Wouldn't the price go down dramatically? The poor/desperate, which would be the ones willing to sell a kidney, would be taken advantage of. How does anyone figure the going price would remain at $50,000+ if poor people will be lining up in droves to "donate"? The price would immediately drop to the lowest possible, as the demand would not match the willing supply, the supply would be saturated.
I don't see a way this could work to equally benefit the recipient as well as the donor. Even with some kind of waiting list or lottery, there would still be more willing donors vs. recipients. And that means lower prices and standards for the donors to expect.