If you allowed people to sell their kidneys the market price would definitely be a lot higher and the trade in kidneys between homeless people and sick people would leave both groups a lot better off. Today you have one group dying in the street and the other dying waiting for a transplant.
And if a few homeless people die from transplant complications, hey they volunteered! And now we can stop investing in social welfare because a steady supply of homeless bodies improves the health of the middle class!
Oh come on, surely you must realize that selling limited copyright to a digital likeness of your face is much different than selling a literal body part.
Of course it's different. It's obviously different. That doesn't make "if it was a bad exchange they wouldn't do it" a good argument in support of either of those examples.