The contributions I've made to my donor advised fund exceeded $50,000. Since I'm already giving away more than $50,000 I wouldn't be interested by an offer of $50,000 for a kidney.
I suspect people are drawn to FIRE because they have incomes and net worth that are in the upper quartile. For people with higher net worth, it is easier to turn down $50,000.
Compare that to people who have a credit card balance but no assets, and they will be far more likely to give up a kidney for $50,000. I'm not sure how to make that fair - to avoid disproportionally interesting people who are financially worse off.
This is addressed in the podcast. Plasma donation is painful, but no one who's sold their plasma when they came upon financial hard times wishes that they
hadn't been allowed to. Some countries ban compensation for plasma donation, and instead of the demand for plasma being met by people donating altruistically, those countries have to import plasma from countries that do compensate donors. So by outlawing it, they're not preventing plasma from being bought and sold, they're simply outsourcing the "exploitation" to other countries, while preventing their own poorer citizens from making a tradeoff they might wish they were allowed to make.
It is akin to compensation for work: more privileged people won't take riskier work or work that's harder on the body, like roofing or picking strawberries. But we don't ban people from working on roofs or picking strawberries, because somebody's gotta do it! A lot of people need healthy kidneys. There could be plenty to go around, if people could be compensated for the time and hardship associated with donating.
Would I donate my kidney for $50,000 in 20 years when I'm in a much more financially comfortable position? Probably not. But would I make that tradeoff right now in the accumulation phase, as a $50k head start in life? Sure. Is it exploitation for an upwardly mobile middle class person with a healthy emergency fund and a maxed out Roth IRA to "sell" their kidney? No, it's just a tradeoff. Which ends up being net positive for society, as it gets someone off dialysis, vastly improving their health and quality of life while costing us less money. If someone who is more financially desperate than me decides to make that tradeoff, I'd say that's even more positive for society, because they benefit from the marginal dollar more than I do.
The other major difference between this and an open market for organs is that all kidneys donated through this system would be allocated in the order of regular donor allocation procedures, not sold off to the highest bidder. The donations might disproportionately come from the less well off, but they would be distributed in a non-market manner, based on health considerations and wait lists and whatever else goes into that process.