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Different people have different methods and sources of truth. My method is just my personal way. At the end of the day, you pay what you feel the card is worth. Do your due diligence.
Have you ever seen a Pokemon card and wonder how much you should pay for it? Maybe you pulled a hit and you're not sure what's a good amount to price it. Or perhaps you saw a card going for a trillion dollars and you're wondering how.
The first thing to realize is: trading cards and collectibles have no inherent value. Or rather, you're not paying for the actual cost of the cardboard, ink, and labor that goes into creating the card.
A card is worth whatever the market is willing to pay for it. And a good source of truth is the last sold price on eBay. This is where most transactions for singles happen, and where you can find real data on. Though there are cases of people trying to pump prices (similar to crypto pump and dumps). So be cautious. Don't just take last sold prices as gospel.
The best method is to take an average of the last few transactions of the card. There are places where you can see a ledger of last sold transactions. Pricecharting and 130pt are a couple of websites where you can find it.
Pricecharting also has average prices of graded cards so you can start there. The problem with Pricecharting is that some older cards or cards which may not be circulated much don't have enough data to get an accurate average.
So some people might not use it as a source of truth for rarer cards.
One method you can use to get an idea of what the market is willing to pay for something is to do auctions in Facebook groups. Set a starting bid (SB), a reserve price (RP) and a buy out (BO) price if you want to.
Starting bid is self explanatory. Reserve price is the price where if the auction reaches that amount, you will be forced to sell the card to the bidder. BO price is the price for buying the card and ending the auction immediately.
Some people set the starting bid at $1 (SB1), RP at an amount they are comfortable letting the card go at, and no BO price, just to see how high the market is willing to bid for a card. That way, if the auction doesn't reach the RP, then you're not compelled to sell the card to the bidders, but you still know what your card is worth (plus minus 10%).
Some retail sellers and vendors at card shows often buy singles at a percentage of market value, and sell them at above market value, citing rental and manpower costs. If you are not in a hurry to get rid of your cards, I suggest you avoid selling to vendors and shops. Instead, hold them until you find a reasonable and fair price.
You can also opt to trade it with other collectors at a fair value. At the end of the day, it's all just collectibles and as long as you do your due diligence and are happy with the deal, that's the goal.