Your argument makes sense when you view such transactions as a decision problem with a single actor and no temporal element. But such transaction happens withi

Your argument makes sense when you view such transactions as a decision problem with a single actor and no temporal element. But such transaction happens within a lifelong game with at least two players. Changing the rule of the game can influence their game tree and their behavior, far before they get into a situation where the change applies. For the kidney trade, banning it might discourage potential organ-buyers from 'check-mating' people into selling their kidneys. It also might nudge parents to work harder since they have one less things to sell off. I acutually have no opinion over banning organ trade. I'm just saying that it could make some sense.