[Bram99]

The problem that has been described here is ancient. In fact, it gave rise to the concept of money as a medium of exchange. It can be substituted for very diverse goods.

The first known form of currency was the Mesopotamian Shekel from around 5,000 years ago. Likely there were similar quibbles to the one in this thread, when a wheat farmer had too much wheat but really wanted some goat milk.

Today, it can be used as a medium of exchange (to buy or sell) a PSA 3 T206 Ty Cobb with poor centering. The money proceeds from a sale like this could be used to buy a different T206 Ty Cobb graded by a different TPG. Or it can be used to even out a trade involving the two cards in the example.

The key issue in any of these examples is the need to come to an agreement on relative value. That problem is also ancient and didn't arise with the advent of price guides, the rise of card "investors" or card dealers, or third party grading.

So if you can't think of a fair trade, try using cash to help even things out! If that doesn't work, you probably weren't likely to have a meeting of the minds in a buy/sell transaction either, so might as well move on and find a new trading partner!