I think it happens en mass. They have hospitals and clinics where people do this in the third world; often with the medical tourism facility right next door. I've met two people here in Thailand who have given kidneys although they did not tell me the circumstances.
Those vigorous rich 80+ year olds in power have to get their new organs from somewhere. The sick fat people dying in the us are not providing healthy organs to the volume needed and only so many young healthy people can be shot in the head.
I looked at the stats a while ago and then numbers for transplants are off the charts. I could not find good data on where all those organs were coming from. Young, healthy, non-drinking people are the best providers of viable organs. It's a nasty subject and I stopped looking after I saw it was going to get worse as I looked further.