> The seller knows the "real" price…

The seller knows the customary price, and the lowest price they would be willing to accept. The "real" price is whatever the buyer and seller agree on for a particular trade. Price discovery is an ongoing process which isn't finished just because you've determined an average price for past trades. The buyer isn't losing out unless there's actual fraud involved, even if they could have potentially negotiated a better price. And really, we're mostly talking about trades between individuals and small businesses (who may also be individuals)—why should all the surplus value represented by the difference between the highest and lowest prices acceptable to both parties go to the buyer, and not the seller?