Analyzing any single transaction, it seems trivial that what benefits a seller (lower price) hurts a buyer and vice versa.

Life isn't a zero-sum game, and individually profit-maximising actions may not be in even the profit maximiser's interest long-term, let alone society's.

In the European Antiquity and Middle-Ages, at least, there were laws against merchants taking too much profit. It didn't matter if a high price optimised the allocation of a resource - the gouging was seen as more deleterious to society than the inefficiency, and malefactors were punished harshly, with heavy fines or even exile or death.

I feel like we're due for a medieval revival.