Edit to state the central point clearly at the top: why do you think restricting their range of possible choices "protects the poor and weak" ?

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.

The core idea is to limit the choices of the rich, not to limit the choices of the poor. You will be prosecuted if you buy a kidney, or if you kill a human, not if you sell a kidney or your life.

With respect to your interpretation of the veil of ignorance, if you knew you were going to be a beggar, why on earth would you want to design the society in such a way that selling your kidneys or life can become the best option left to you (or anybody else in that society)? And if you are able to design it in such a way that it is not the best option, then outlawing to buy such things would not impact your life as beggar negatively.

From your reply I think you did not entirely understand the parent's reasoning. I think you don't follow the consequences of your points to the end, where it becomes visible that they are not desirable. I believe that is what the parent is aiming at.

Instead of just saying that it's obvious, it's visible, etc., can you explain the reasoning?

One scenario is easy to figure out: Once it's simple for poor beggars to sell their kidneys, it becomes normalized and common. It's expected that you'll sell your kidney or you clearly don't need help that badly. The price drops from abundant desperate supply until it's only enough to live on for a few months. A few years down the road and you have just as many poor beggars but all of them are missing a kidney and less resilient to illness.

The root problem here is that if people are put in desperate situations, some amount of choice is forcibly removed from them. Certain agreements can't be fairly negotiated unless you remove the desperation first.