Based on the conversation happening in this thread…
Should all comps be treated equal?
What if they are outliers (e.g., extremely high due to shilling; extremely low due to the seller not understanding market value)?
How should buyers/sellers best use sales data?
no, past comps dont matter for rare collectibles unless its some modern junk 
Sold listings are data and some data is better than others. When there is a lot of data, the buyers have the power in the negotiation. When there is little data, the sellers have the power.
In situations where the market has not dictated the price through enough sold listings, the seller gets to choose the price.
This is a very complex question and my answer depends on exactly what you want to do with “comps.” Forgive me for the long-winded analogy, but let’s compare Pokemon to the stock market. In the stock market (or a major stock exchange to be more specific), you have effectively instant liquidity. Because buying and selling is going on all the time, you can easily establish an exact market value for a particular stock. That value fluctuates up and down as necessary, but the volume of sales is what gives the stock its exact market value.
Bringing this back to cards, “market value” becomes more established the more sales of a card you have. If there are 500 of a particular card listed BIN on eBay between $150-200, and auctions sell consistently for $130, there’s a clear established market value for the card. If a seller and buyer are looking to make a deal, using “comps” in this case is helpful because you have an established floor price (auctions) and a ceiling price (lowest BIN) within which to negotiate. In my hypothetical example, that gives you a bit of room but not too much. If there are outlier sales, these are easily recognized and are folded into the calculation as noise.
However, if there are few to no copies of a particular card listed, and little sales history, “comps” mean much less because the sales prices are susceptible to outside influences (person needs to get rid of the card fast, person is highly motivated to acquire the card, outside actors wish to inflate the price, etc). With few copies available, the buyer and seller’s individual circumstances matter a lot more. It’s harder to establish a “market value” because of this.
I would go as far to say that for rare cards that transact infrequently, “comps” are just a narrative tool used to justify a price rather than reflecting anything inherent about the market.
Tl;dr what pfm said but in more words
This is the big one. Comps simply exist, and there is a logical way to interpret them. However people often misinterpret comps unknowingly or intentionally out of self-interest.
I’ve discussed with sellers about buying some rarely seen cards and occasionally there has been some dirt cheap single comps with those sellers clearly not knowing what they are selling, i’ve straight ignored them while trying to figure a price to agree on with seller. That data is completely irrelevant with a seller that knows the actual approximate value and it would be childish to even try to mention them in a serious way
No, you need to find a median and some comps can skew the data
I always look at last solds for general comps as well as the supply on the market. I disregard auction prices in general for comps (as a seller). If I’m the only one on the market comps go out the window and I’m crankin the price and let the market decide if its too high.
I didn’t read the conversation this spawned from, does anyone here actually believe that all comps are created equal? Can anyone argue that?
It used to be (back in my early days) “comps” were rarely looked at as most people didn’t even know you could pull completed sales data off of ebay. (It existed, people just didn’t check it). Now there are a MILLION tools to look at where cards sell at, especially for modern stuff.
What I’ve found is that “comp” data is typically misconstrued for personal gain, especially among buyers. I’ve had multiple buyers “pull comps” and given me wild numbers. I’ve quickly found and quite frequently that they have usually done one of the following:
In every instance of the above, they then find that the number they have is “right” and should be the price of the card I’m attempting to sell. And more often than not, if I refuse the number given (I usually do) I’m treated like an unreasonable sleezy car salesman trying to squeeze every dime out of an extended warranty.
What I will say about comps is that they have a place and that is as a data point. You can use it to figure out every sale, or not. While inherently nothing wrong with citing comps, I would not say it’s actual market value. I typically sit on cards and sell above comp prices depending on the card. For me comps are just a “if I want to sell it today I can probably get this much” guide.
In my world it almost always makes more sense to sell to someone who is a collector or speculator as they will pay more than a “today” price. I’ve bought cards above market as well, as I’ve speculated too. Does that mean my comp rate is equal to ebay data points? Not at all, I live in a different world. So to answer the question, no comps should never be treated equal there are too many various factors.
I just scrolled through the linked thread but I’m pretty tired so I’ve probably missed some context from the previous conversation and/or why it spawned a second thread.
I guess the short answer is data is data and it can only be interpreted. Whether it should be treated equally is up to the interpreter(s).