There is no reason that he =should= be buying it for exactly the same
price.

The idea of a fixed price, as opposed to each transaction involving
negotiation, is a relatively recent innovation. How this innovation has
become a Law of the Universe, I cannot say.

Somewhere I read that it came about in the Puritan days. Or Pilgrim
days? What's the diff between them, anyway? The story is that the
leader of the group got tired of all the time wasted dickering about
prices on market day and persuaded the people to set fixed prices for
goods.