system
A chat on subreddit /r/NoStupidQuestions
human
Also people would straight up murder people/force others to sell their organs. 

If you had liver failure and were dying waiting for a transplant and you "knew a guy who knew a guy" who could get you one for your life savings--no questions asked... would you? More of us would say "yes" than you'd like to imagine. When that market is there there will be people taking advantage of it.

Making it illegal to accept money removes ANY room for profit. Businesses would be able to be punished without identifying victims.

Also a lot of illegal organs (which is a bigger industry than you'd expect) go for hundreds of thousands while the poor donors get thousands, if they're compensated at all.
gpt
See china