A common complaint about a market in kidneys is that the sick poor would not be able to get kidneys as easily as with a first-come, first-serve system. Even the lowest estimates of the price of kidneys in a free market is a significant sum for many of the world's poor. But this argument ignores one of their biggest assets — their healthy kidney before the onset of failure.
Most underlying causes of renal failure affect both kidneys, so keeping a reserve kidney in case one fails will usually not work. Also, kidneys can only be stored outside the body for about 30 hours, so storing your healthy kidney for future transplant is impossible.
In a sense, though, it is possible to store your kidney: in a market for organ transplants, your healthy kidney can be sold and the money earned from the sale used for another purpose, such as the purchase of another kidney in the future.
One way to help ensure you will have the market value for a kidney in the future is to sell one now at current market value and save the money. Prohibition of the sale of kidneys takes away the one asset that most of the poor have that could potentially save their lives.