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.
We shouldn't allow people to get into a situation where they have to choose between losing a kidney and their children starving.
But - if someone is in that situation - it makes absolutely no sense not to let them decide which option they find least worst.
By doing that, you're only making things worse.