In regards to both prostitution and organ donation, there are concerns about the pressures which lead to the "choice" to sell one's body or parts of it thereof. The economic pressures can take away the element of choice. The bans on selling both is based on the idea that in many, if not most, cases there is not choice but desperate need. The wealthy should not be allowed to exploit the poor and weak, who cannot defend themselves. The individual who faces enough economic opportunities so that those are only one of many options is too rare, when looking at who partakes in the activity, in my experience in this state in my own world.