For one, most organs can't even be donated during life. Paying people for them is useless, because they're dead. But an approach that has always appealed to me, is to give people who are registered as donor preference when they need a transplant. And that means people unwilling to donate their organs when they die, are also less likely to receive one.

Children should be exempt from this of course; they can't make these decisions yet and should always get preference, but I've always wondered why someone who doesn't want to donate their organs, should have a right to receive an organ.

It feels more just that people willing to help others, should also be first in line to receive help. It's a practical application of the Golden Rule. And this system would also mean that rich people are just as likely to register as donors as poor people, without any money needing to be involved.

Blood and kidneys are different of course, because they can be donated while you're still alive. I personally struggle to imagine donating my own kidney while I still need it. What if something happens to my other kidney? So people who have donated a kidney should absolutely be first in line to receive one should their remaining kidney give out.

As for blood, paying for it makes sense to me. You don't cripple yourself long term, you're not giving anything up permanently. Lots of work is less healthy than donating blood. Paying people for backbreaking work that will likely cost them their health is grosser than paying them for blood donations. And if poor people are more likely to make use of this, I have no problem with giving poor people an extra way to make money.

Although we should definitely ensure sufficient living standards for everybody so nobody would sell body parts out of desperation. That is still the grossest thing.