I guess in all the cases I've traded something, it was just based on both people being happy with the trade period, vs. trading dollar for dollar or value for value.
Recently a local dealer had a piece of early Marathon Ethyl ad glass I wanted. He had $49 on it in his shop. I took him a box of cans....duplicates, others that came with lots or groups that I didn't want, etc. After we traded my box full for the one piece of ad glass, I went back to his shop a month or so later. While many of the cans I traded were already gone, I saw a couple that he had priced at $29 each, another at $39. I'm sure he made out pretty darn good on the box, but did I care? No, I was happy with the ad glass piece I got, and made a bit of room in the process! I was happy...he was happy...good trade.
Darin