I think the point was that it is hard for participants to cheat, the owner isn't going to go lower than a certain amount; thus, the owner is always going to break even or profit. If the owner puts the price too high then the buyer is no longer going to be a buyer, but a a person who is walking away from the deal. Kind of self-enforced because the person who is selling is not going to make money by putting too high a price on goods.