There's also strong arguments for prohibiting the sale of human organs, chiefly that you want to avoid pressure against poor people to give up their kidneys to survive. Even for something as simple as blood donations, once money enters the equation, everything changes incentive-wise... you don't want people to lie on donation forms about how long ago their last donation was just to make money (and then end up being carried off to the ER because they gave up way too much blood).

The other debate that inevitably crops up is ethics, similar to the debate around surrogate pregnancies, we are already seeing women from piss poor countries taking up one of the most serious health challenges there is for rich Western women - women that just don't want to risk their bodies getting fucked up permanently by pregnancy.