That's a simplifying assumption used to make the accounting practical. But knowing that someone bought something on Amazon doesn't tell you how much the buyer

That's a simplifying assumption used to make the accounting practical. But knowing that someone bought something on Amazon doesn't tell you how much the buyer would have paid or how low the seller would have been willing to go. (There are gains from trade and who gets them depends on negotiations and the competitive situation.) Also, one or even both of them may have made a mistake. People return things they purchased all the time, and they may regret purchases they don't return. We are fortunate that for many kinds of goods, we usually don't have to pay the full value of what we buy, due to competition.