It's unlikely there would be end-to-end oversight.

Why not? It's outright illegal in every country except one. I would expect it to be heavily regulated if it is ever allowed. If there are problems with stolen organs, they would definitely start regulating it.

Also, I haven't seen anything saying that this is a problem in Iran. I read it has been a problem in India, so they made it illegal. It's not the same as regulating, but it shows that they won't allow that sort of thing to happen.

I disagree with your characterization of pressured people as idiots.

I just mean people selling their kidneys when it hurts them. "Idiots" meaning acting irrationally, not necessarily stupider than average.

I guess it's possible for it to happen like you describe so that they have an additional incentive to sell, in a bad way. I don't think that's likely to happen.

I don't see at all how a price floor helps with that.

It makes it more likely that it actually will be worth while for them. If it's never worth while to sell your kidney for below $x, then setting a price floor above $x would at least mean that some of the people selling their kidneys would benefit.