Earlier quoted context omitted.
You first -- on what principles do you think the grandfather's choice in your example should be illegal?
Illegal simply because it is murder, regardless how much money the murderer pays in advance. Immoral because human life and dignity should not become a tradeable commodity. A society allowing this would quickly deteriorate into a system where the rich just buy the desired behavior from the poor, and we would end up with an oligarchy instead of a democracy. Another angle to object would be Rawls's theory of justice: s…
If you have a shitty job, would you be better off getting fired? No, because presumably you already had the ability to quit. (Anticipating someone jumping on me for the analogy: I'm obviously not saying having a shitty job is equivalent to being in such dire straits that someone would consider selling their kidney. I recognize the a huge difference in degree. I'm just illustrating the point.)
> Immoral because human life and dignity should not become a tradeable commodity.
This is just restating your conclusion, not providing an argument.
> A society allowing this would quickly deteriorate into a system where the rich just buy the desired behavior from the poor
That's already the system we have. I can entice people to do all sorts of things that they would rather not do, because I have money, like cook food for me, build airplanes that I can fly in, and so on.
> and we would end up with an oligarchy instead of a democracy.
We're not talking about making it legal to spend money to influence voting or politics, so I don't follow this.
> How would you design a society under these conditions?
My argument doesn't even rely on this veil of ignorance! Even if I KNEW I was going to be reincarnated as a poor beggar, why wouldn't I design society to give myself more choices, rather than fewer? It's not like anyone would be forced to sell their kidneys if they don't want to.