One scenario is easy to figure out: Once it's simple for poor beggars to sell their kidneys, it becomes normalized and common. It's expected that you'll sell your kidney or you clearly don't need help that badly. The price drops from abundant desperate supply until it's only enough to live on for a few months. A few years down the road and you have just as many poor beggars but all of them are missing a kidney and less resilient to illness.

The root problem here is that if people are put in desperate situations, some amount of choice is forcibly removed from them. Certain agreements can't be fairly negotiated unless you remove the desperation first.