To add onto this —- living with only one kidney rather than two only barely increases your risk of dying, and many people each year die due to lack of kidneys. So, creating a market here (regulated of course) would save a lot of lives.
For anyone who thinks this is hyperbole, ProPublica has done excellent work investigating how lenders use the court system to imprison Americans who can't pay back their debts: https://www.google.com/search?q=propublica+debtors+prisons
Debtors prisons are bad but of course that’s a separate issue.