Either regulate the price of organs, or not allow people who are desperate (could be defined by income or wealth) to sell their organs.
We already do that. We regulate the price at $0.
Actually, there are two ways to limit it. If the price is very low, such as $0, desperation would not affect people's willingness to sell their organs because the price is so low that it can't relieve desperation. If the price is very high, desperation is not making people sell the organs for a lowballed price. It's prices in the middle that are the problem.
(We also do this with normal labor. You can give away your labor for $0, and you can work for an amount that is at least the minimum wage, but nothing inbetween.)
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.