I disagree vehemently with your viewpoint. I fear that you've been trapped by the vivid image of an impoverished, innocent, victimized person being blackmailed in a dark alley into selling their kidneys (picture someone like Fantine from
Les Misérables) and try to stop that at all costs. That's wrong, though: Nothing is worth stopping at all costs, that denies the meaning of the word "cost". We need to objectively weigh the costs of that image against the vague tragedy of early death in a hospital bed multiplied by the 18 million people currently dying in need of a transplanted kidney. And added against that sum, consider that the vast majority of donors would not be victims at all. A few points which I think deserve consideration are:
> This is not an easy subject, but I think there is nothing immoral about letting a human being die if their organs fail...It can be thought of as being sad. But not immoral or evil.
It is most certainly evil if there is a perfectly good spare kidney just sitting around in a nearby abdomen, and someone dies because they don't have access to it. Human life is intrinsically valuable because of the consciousness and experiences of the mind which is enabled by functioning organs. If the organs cease to function, that does not reduce the value of that life, or imply that the 'time was right': every death is a tragedy.
> Also, since death is inevitable, I think in most cases, it is better not to drag it out.
Death is temporarily avoidable (evitable?). It cannot be infinitely extended (yet), and you can talk about weighing the costs of people suffering for extended periods of time at great cost in hospital beds or from mental illness against the happy people who made unlikely recoveries....but this weighing does not reasonably extend to kidney failure. Kidney replacement often takes people who were otherwise healthy and happy from suffering an unnecessary early death into many more years of happy, healthy life. When death can be avoided with this level of success, it should be.
> The thing is, for most people, leading a poor (as far as you are not starving) life is better than having some temporary prosperity by selling a kidney and leading degraded life till the end.... Now. That is evil.
Three responses to this: First, this will not be the epidemic you describe. Second, the life of the donor is not degraded as you describe. Third, consider the recipient.
Demand in this market will have a very small cap. You can't sell more kidneys than there are people who need transplants. There are currently about 20 million people worldwide who need transplants, so only 1 in 350 people will have an opportunity to profit from this market. You won't have a large class of society who depend on the ability to fund their children's education with one parent's kidney and buy a retirement home with the other.
The donor of a kidney will be unlikely to suffer a degraded life. Most people really don't need two kidneys to survive. Kidney donation is a very safe procedure, and donors typically live a full life after donating. It's prudent to perform regular check-ups, and there is a slightly increased risk of needing a transplant (after all, you only have one kidney left!) - but in a world where kidneys are available to those who need them, that's less of a problem. Also, remember that temporary prosperity used to, as you describe, fund an education may lead to permanent prosperity, and that poverty often does result in a reduced lifespan and sometimes in starvation.
Finally, even though these donors are a small population, are not negatively affected by donating and could in fact be positively affected by it, consider the recipients. Even if there were a significant negative result to the donors, it might result in more positive utility overall. The people in need of a kidney could, in one world, suffer in a hospital bed on a dialysis machine for the short remnant of their life. In the other, they could live many more happy years with a transplanted kidney.