Of course you can, the law doesn't have to be simple, carve outs are perfectly okay, if that is what the people (seem to) want. That said there is the very sim

Of course you can, the law doesn't have to be simple, carve outs are perfectly okay, if that is what the people (seem to) want. That said there is the very simple argument that selling your kidney should be allowed. And if we are worried about someone exploiting others by forcing them to sell their kidneys then we can regulate the commerce part (set minimum price, blind donors to recipients), just as the medical market is regulated. (And in general if oppression/exploitation is a concern then the law should address that - provide a social safety net so people don't have to even think about to sell their kidney for money, instead of regulating the symptom.)