Why not? That's a serious question; I'd love to hear your opinion.

It seems to me that if they freely consent to sell their kidney, then they'll only do it because it's a better option than not doing it. Sure, in a perfect world everyone would have enough money to survive, and there would be no need for this. But is it really ethical to deny them options?

But you are ok with the "Freedom" to die on needless waiting lists, while willing donors would be available if payment was possible?