Sold comps, matching printings, dealer rates, and raw against graded, in the order they actually matter.
Most trades at a show are settled from memory and a phone calculator. That is fine when both people are guessing by roughly the same amount. It stops being fine the moment one side has looked something up and the other has not.
Here is the order to work in. It applies to Pokémon, to One Piece, and to any other card game where people quote each other numbers across a table.
An asking price is one person's opinion. A sold price is two people agreeing. They are frequently not the same number, and the gap is widest on exactly the cards trades get argued over: the hyped ones, where hundreds of copies sit listed at prices nobody has paid.
So when you look a card up, filter to completed sales. On eBay that is the Sold Items filter. In a pricing catalog it is whatever they call market price, which is normally a recent sold average rather than a listing average. If a tool will not tell you which of the two it is showing you, that is worth knowing before you trust it.
Ignore the outliers at both ends. One person overpaying at 3am is not the market, and neither is one auction that ended at a bad hour.
This is where most honest mistakes happen, and it is not a small effect. The same artwork can exist as a common, a holo, a reverse holo, a first edition, a promo, a Japanese print, and a modern reprint, and those can be worth wildly different amounts. Four things have to match before a comp counts:
If you cannot confirm all four, you do not have a comp. You have a card that looks similar.
A slabbed card and a loose copy of the same card are priced by different people for different reasons, and the graded price is not the raw price plus a fee. It reflects what buyers pay for a condition somebody else has certified.
Two rules follow from that. Never price a raw card off graded sales, which is the more expensive mistake. And never price a slab off raw sales, or off a different grade: a PSA 9 and a PSA 10 of the same card can be a long way apart, and on some cards the distance between them is most of the value.
If a card in the trade is raw and one person is quietly imagining it graded, the trade is already unfair and nobody has said anything untrue yet.
Nobody trades at full market. A dealer buying your card has to resell it, cover the booth, and carry the risk that it sits. So they buy at a percentage of market, and that percentage is the thing people are really negotiating when they argue about a card.
The number itself is a matter for the two of you. What matters is that it is stated out loud, and that it is applied to both sides of the trade. A rate applied to your cards but not theirs is not a rate, it is a discount, and it is the single most common way a trade goes sideways without anyone lying.
A worked example, with round hypothetical numbers. Your side totals $400 at market and theirs totals $380. At an agreed 80% rate that is $320 against $304, so the gap is $16, not $20. Applying the rate to one side only would have made it look like $80.
Cash added to one side is worth exactly its face value and is not subject to anybody's buy rate. It is the cleanest thing in the trade, and it is the usual way a small gap gets closed. Write it on the side it is going to, not in your head.
Two sides of a trade almost never land on the same number, and they do not have to. What matters is whether the gap is small enough that it is noise rather than a transfer.
Bartr's rule is that a gap under 3% of the larger side is fair: close enough that a few dollars of cash or a filler card settles it. Pick whatever threshold you like, but pick it before you see the number, because a threshold chosen afterwards is just the answer you wanted.
Trades get remembered wrong. Not dishonestly, just wrong: a month later two people recall two different sets of cards, and neither can check. A list of what each side gave, what each was worth on the day, and what the gap came to takes a minute at the table and settles every version of that conversation.
It is also the only way to find out whether you are any good at this. A trade that looked fine can look different six months on, and you cannot learn from a trade you did not record.
Doing all of that by hand, per card, across a table, is the problem. Bartr does this arithmetic while you stand at the table: it prices both sides of a trade from sold-market data, applies whatever dealer rate you agree on, and writes the result down so you can check it later.
Join the free beta, or try the trade calculator in your browser first. Neither needs an account.