1- I presume it refers to the donation of organs rather than the sale.
2- It shows that the author wants to argue the idea of paying being wrong. Buying organs is possibly preferable to watching a patient die.
3- Presumably, tghe doctor didn't speak the language of the donor, so showed him some money to try to see if he had been paid to donate.
4- should sell = chooses to sell
5what you will = what you want