I think this really depends on a lot of things. I haven’t been holding sealed product and what I do have, I generally intend to keep in my collection, so just take what I am saying with a grain of salt.
You kinda have to decide where/ what margins you’re comfortable selling for. 600% on modern sealed sounds like a done deal to me, but I don’t really have interest in holding loads of boxes. So, I’d probably be more happy putting that money elsewhere.
As for what Scott says; I am not sure modern sealed falls under that ‘difficult to acquire’ umbrella. Again, it depends on what you have (English, Japanese, Booster boxes, ETBs, collection boxes, etc.) I think what he means is to consider that all items are not the same. For example, I am probably not gonna sell my PSA 10 2019 Player’s Ceremony. They’re definitely worth some money, but sourcing mint condition copies to grade is too difficult and costly for me to replicate. I’d feel more comfortable selling an unlimited base set holo because there is more of them and if I really wanted it back, I could probably make that happen. I think Scott was referring to much more rare items; if I had the money, I am sure I could probably reacquire another copy fairly easily of Player’s Ceremony. However, flipping a 2001 Tropical Wind is a completely different story. That is an item that, regardless of how much money you have, may simply just not be available to purchase. That’s an extreme example, but each item has it’s relative scarcity and rarity.
Also, I just want to add; Scott sells cards. There’s items to sell and items to keep, but that’s really going to come down to what you value in your collection, or which items you have confidence in. I think the decision to sell can be a fairly natural process, but it could also be motivated by a need for money. What I got from his statements was to not put yourself in a position where you have to sell; meaning, do not overleverage yourself and get into a spot where you have to let go of some rare, good items, to cover rent or something. You could always sell some of what you have to supplement the purchase, doesn’t have to be all or nothing!
At the end of the day, we’re all gonna make mistakes and that is okay. These decisions are an opportunity to learn the answers to your questions. Don’t let the fear of striking out keep you from playing the game!