What you're missing is that China is already executing prisoners in order to harvest their organs. This isn't hypothetical. So anyone who wants to advocate for selling human organs for transplant need to address how to avoid abuse of powerless victims who may be compelled to donate.
Speculative? They're using kidneys from prisoners, once they are executed. To turn around cause and effect is disingenuous. You can guess that that may be happening; to state it as fact is quite a stretch.
Step 1: Reward volunteer donors with cash, healthcare
Step 2: Reward 'volunteer' donors with welfare, housing
Step 3: Reward 'volunteer' donors with reduced criminal penalties
Step 4: Reward 'volunteer' donors with reduced prison sentences
Step 5: Make donation a compulsory sentencing option
Step 6: Compel criminals to donate both kidneys
Now you're China.