How is it exploitation to legalize more types of consensual transactions?
The law, in its majestic equality, allows rich and poor alike to sell their internal organs to try to make their power bill payment next month.
That's without explicit coercion (sure, I'll do X for you, so long as you sell your kidney).
We all buy things daily from entities with much more power than ourselves, for example.
Hypothetically, if someone wants to sell a kidney to pay the bills, and they are of sound mind, the government shouldn't stop them.
That is rhetorical, it is highly unethical. And similarly, desperate people will sell organs under financial pressure and limited or no regulation on the sales tactics or accurate disclosure.
A more effective plan would be to offer $100-$1000 cash payment at time of registration when getting a license to be an organ donor. It is hard to find matches. Better and more economically efficient to have a broader pool of potential donors, for all parties. And since the donors are already brain dead, no ethical dilemma.