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.
