In terms of pure ethics/morality I agree wholeheartedly with @churlocker
OP was never asked for their opinion on the price, so is not obligated to give one. I do think the relationship with the seller should be taken into account as well though. Would you have done the same to your siblings? Best friends? Friends? It’s up to the individual to find where to draw the line.
If he offered it at $30 that lessen any guilt.
Other points to consider:
In order to justify bad behavior some here are over complicating this. Don’t pay attention to it.
It’s simple. Your co worker didn’t know the value and that opened up a great opportunity to help him OR take advantage of him. The benefits of helping someone else far outweigh the dollars you’d profit otherwise.
Instead you chose to take advantage of him and that was not the right thing to do.
Scott’s point if on eBay wouldn’t apply here. This was someone you know. You were even in his home.
A gauge I use often is, if my mom was still alive “What would she have me do?” And I know exactly.
Of course you’re not a bad person. You simply did the wrong thing. You can fix it though:)
In order to avoid having to critically think about why they behave the way they do, some people are over simplifying it. Don’t pay attention to it.
It’s simple. Your co worker didn’t know the value, yet asserted one arrogantly, and that gave you the opportunity to give him exactly what he wanted. The benefits of someone’s confidence can’t be weighed against the benefits of their own financial gain. You gave him his confidence and that was the right thing to do.
Scott’s point if on eBay is exactly the same thing, as we’ve already stated you aren’t ethically justified in treating a stranger any different from a friend.
A gauge I used often is, if my grandmother were still alive “What would she have me do?” And I know exactly. She’d have me use that education and think critically.
Of course you’re not a bad person. You simply followed the situations logical conclusion. You have no reason to fix that. 
Passive aggression and simplification don’t change reality. They just make you look unable to explain yourself. I’d be glad to give insight on why I’d opt to act differently than you are ethically obligated to, should you care to hear it, 2htu2. But if you take anything from this, take away that it’s good to know why you believe what you believe. It’s as important as what you believe. And inconsistencies are all around us. Different perspectives exist in people that we love, trust, and should have confidence in. That’s just life.
Don’t be arrogant when you don’t know about something and you’ll never run into this situation. That was the true downfall of your colleague.
Stop trying to find a “one size fits all” conclusion, because that is not how the world works.
In this instance the OP felt the need to question his purchase here, even the title points to them thinking they have done something morally wrong.
We can go by all the “what if” solutions all day long. They aren’t relative nor even important to this discussion. We have the scenario and the circumstances.
Who even cares what any of us think. OP made the deal. It’s done. All that matters is what OP thinks now.
Who cares what any of us think?
The whole point of this topic was literally the OP asking us what we would do and what we think of him… lol
Yet you have people willing to inject irrelevant points to the discussion. All these what ifs.
My point is simple. OP shouldn’t care what we think.
That too hard to understand?
Again, I just reject this type of attitude altogether. I’m glad to respectfully agree to disagree, but if you’re going to argue that situation by situation you get to act differently, why are you the one that gets to determine how the right way to act is?
Unless you’re appealing to a standard of some sort. And then, surprise: we’re at square one. Define the standard. Provide the guidelines. Because there has to be consistency or, quite literally, we’re just making it all up as we go. And then, right and wrong is out the window.
Again, I reiterate that personal conscience is a horrible indicator because conscience is informed. It doesn’t just appear. It’s why we can have child soldiers who don’t understand what they’re doing. And who is informing that conscience is of the utmost importance. I’d prefer that be a critically informed conscience. So when OP asks an ethical question, I’m going to see critical advice to inform them.
I have no interest in the what if solutions. I’m talking about this problem right here as OP stated it. I’m not interested in addressing it any other way. But of course, it’s all connected in one way or another because how we act in one situation has implications for others. Its why systems exist and why a code is necessary. They are relevant and of the utmost importance in a conversation such as this. Because we have a scenario that will be replicated with very common circumstances.
EDIT: To address the above that was posted before I had submitted: OP should care what people think so long as they can justify why they think that. If we reject what everyone else thinks, we’ve lost community, reason, society. That would be altogether impractical.
I agree, I also pointed out the irrelevant points, the reason the hypotheticals are brought up is just because it’s a fun discussion and its debate tactic people love to use; however, irrelevant.
Just because you believe the OP shouldn’t care what we think, doesn’t mean he won’t care. That’s the difference.
He was asking for suggestions. Whats the point of suggesting he should not care what we all think even after doing it. His ultimate decision is implied at the end anyways. It’s basically like him asking for directions A or directions B and you answer “it doesn’t matter just pick one” haha
But I’m not the one here trying to morally or ethically justify certain situations like you are mate.
I’ve offered my piece on how one should feel; not based on total strangers input to a moot discussion.
If it is so hard to realise and comprehend what I have said then I respectfully bow out of this discussion because I have nothing further to add.
We all have our own moral compass. If there was nothing wrong with the deal, OP wouldn’t have posted here. I don’t blame OP nor think any less of them for it either. It is what it is. I’m not phased at all.
Im not phased at all either, but you saying " he shouldn’t care what we all think " literally added nothing to the discussion.
But neither does our back and forth, so I respectfully digress and back out.
I do agree with you ultimately at the end of the day though haha.
I agree about not being phased in that I separate this conversation from how I might emotionally fell about any participant. That should always be a given when we’re trying to think through things. But you’re reminder about that is definitely helpful.
I don’t think that asking implies moral incorrectness though. I think it implies wondering. It implies there is formation occurring and a fork has been met. I only want both sides of that fork to be very clear before OP makes the decision. What decision they make is up to them. That I will also grant you.
I also think that it should be made clear that you are very much trying to justify as well as I might be. Any time you pick a side you are justifying your side as right or wrong. Perhaps not clearly or well (not implying this to be true of you in this case), but when someone picks a side justification has occurred by virtue of their persuasive involvement.
I also disagree that the conversation is moot as I’ve reiterated many times why I feel conversation like these have gravity and ought to be taken seriously.
But I’m happy for you to have participated even if you have chosen to back out now. The conversation is better for having heard from you.
Churlocker@ I believe you should voice your opinion on this (and you already have with 10 posts here) but trying to bait those of us who see things differently is really unnecessary. If the OP only wanted your opinion he probably would have privately messaged you.
Another angle.
If you were your colleague what would you like him to do in this case?
They still teach the Golden Rule don’t they? 
Yeah but Gary, I wouldn’t want someone to expect me to be lazy and have a sense of entitlement to the point where I need the buyer to inform me of a wrong price that I’m valuing an item be.
So we disagree even about the golden rule lmao
You’re so right on!!!
I think that this discussion has elevated to a new level, to the point where it could be helpful for each side to present its argument that way if someone new is reading this who may be in this exact situation in the future would know how to react. Each side presents it argument, that way the individual themselves can figure out what is the best action to take. Taking things too personal would take away from the helpfulness of this discussion.
The reason the hypotheticals come up is because people like to question the consistency of the other person they are debating on an issue. That is why the common question for the people who think OP’s action was unethical ask the question “Yeah, well would you do that you a family member or friend??” Personally, I say it doesn’t matter if its a friend, spouse, family member, co-worker, or stranger. Its not the person that matters, its the situation. Every situation is different. It’s just easier not to take a good deal from a family member vs. a stranger because you know everything about the family member. Like take this for example: If my brother was selling me the card for $30 I’d take it if I knew he was doing well on his own, had a steady job, happy life, and if I had something I could give him for really cheap like a Beatles album he would value way higher than I would. But I wouldn’t take the $30 card, if I knew he was unemployed and needed money desperately. You see how hypotheticals can broaden the entire scope of this whole situation.
But like we said many times earlier, there really is nothing unethical about this situation:
Consenting adults agreeing on a price and completing acceptance on a valid offer.
The people who are arguing against OP’s action are just trying to fill in blanks of the above^ statement to justify their side.
There’s a lot more to get into, but ill pause here for now
Seems so lol.
Actually, you can’t be an expert at everything and hopefully you’ll deal with more people who are ethical than not. I’ve bought and sold many things in my life and simply believe you’ll come out ahead by being truly helpful to those with dissimilar areas of expertise. Maybe not with every person…but overall.
Sure, on a personal level I 100% agree. I even ran into someone who had an estate sale where I found some no-rarity JP. Base cards. I asked him if he had anymore of these with the missing symbols. I told him that they are worth more than what he is asking for, and would pay the price. He ended up applauding me on knowing this knowledge about the cards, and said he would honor his original price because in the end to him they are just $1 cards he knows nothing about, nor ever wants to. This situation could have resulted in a million and one different ways.
At the end of the day, just because you and I would disclose the information on the value of the cards, doesn’t mean someone who wouldn’t disclose that information is morally unethical. If we are talking about value in dollars amount, then time is money remember. And why should some who spent a lot of time and resources be obligated to inform the other party of information they didn’t bother to research on their own. You are using the golden rule here saying because if you look out for a coin collector when they’re selling a $400 charizard to you for $30, that they will look out for you when you are selling them a $400 coin for $30. I feel generally that this is a nice idea, but its one thing to go into a coin collectors shop asking how much a coin is worth versus going into a coin collectors shop saying “You can’t trick me, I know this coin is worth $30 do you want it or not” I just can’t understand where it would be ethical to disclose to him the real value after he made not just a statement, but an offer and did not even ask a question about the coins value.
Not disclosing information does not equal being dishonest. I think that is the fundamental source of disagreement here (?)
Well put Reina. I’d like to hear your second point.
Rich@ yes, we’re aligned morally for sure.
I just have trouble separating business morals from personal ones. And I wouldn’t want to either.
I guess my bottom line is: you will come out way ahead, both personally and business wise, by treating everyone fairly. I’d rather deal with somebody like me than anybody else;)
The purchase is complete and it sounds as if your conscience is bothering you. As stated earlier it does not what we think. It matters what makes you feel good.
You do not have to tolerate us, but you have to tolerate yourself. If that card is bothering now, how do you think you will feel 10 years from now when you look at it?