To each person participating in the transaction, it's a net zero transaction. Person A values the item they're getting more than the item they've giving, but th

To each person participating in the transaction, it's a net zero transaction. Person A values the item they're getting more than the item they've giving, but that also means they believe that Person B is getting the short end of the stick. So +1 for Person A is -1 for Person B. However, Person B obviously feels the opposite, as if +1 for Person B is -1 for Person A. An outsider, Person C, might say that each of them has given up a fruit (-1), and gained a fruit (+1), so both Person A and Person B end up at 0. This is the point the writer was making.