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

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.