Selling a kidney: would the option necessarily be beneficial?

This is the question asked by Timothy Taylor at this blog Conversable Economist. Important here is the claim, popular among economists, that offering people an additional option--in this case to sell a kidney--must make the people better off, because they don't need to choose the option, but if they wish to do so, they can. That is to say, the basic idea that trade is voluntary.

Taylor is responding to an argument by Simon Rippon against this idea,

[...] that when an option is available, at least some people will find themselves under social pressure to select that option, or will be held responsible for failing to choose it. "For example, imagine a cashier at a rural filling station that is potentially vulnerable to an overnight robbery. It may be better for the cashier to have no key to the safe (and to have a prominent sign displaying that information) than for the cashier to have the key which gives him the option to open it. Possession of the key would make the cashier vulnerable to threats, and the filling station worth robbing."If selling a kidney was a legal option, Rippon argues:

"This means that even if you have no possessions to sell and cannot find a job, nobody can reasonably criticise you for, say, failing to sell a kidney to pay your rent. If a free market in organs was permitted and became widespread, then it is reasonable to assume that your organs would soon enough become economic resources like any other, in the context of the market. Selling your organs would become something that is simply expected of you as and when financial need arises. ...

We should ask questions such as the following: Would those in poverty be eligible for bankruptcy protection, or for public assistance, if they have an organ that they choose not to sell? Could they be legally forced to sell an organ to pay taxes, paternity bills or rent? How would society view someone who asks for charitable assistance to meet her basic needs, if she could easily sell a healthy ‘excess’ organ to meet them? ... Wherever there is great value in not being put under social or legal pressure to sell something as a result of economic forces, we should think carefully about whether it is right to permit a market and to thereby impose the option on everyone to sell it."

These are interesting arguments but do they not apply to any number of things we already allow to be traded, eg in the US blood, tissues, sperm and eggs? Do these bad things happen in these other markets? If not, why are kidneys different? If so how are they dealt with in these other markets? Surely the question is one of balancing the costs and benefits of allowing trade, not just pointing to a few possible bad outcomes and banning trade on that basis.

Taylor makes two points in the conclusion to his post,

First, at present, the main source of kidney donations is people who die unexpectedly, with a few voluntary donors. In the meantime, thousands of Americans die every year awaiting a kidney transplant. I can easily imagine that a substantial group of healthy people might not be willing to donate a kidney for free, but would be willing to do so for substantial compensation, and encouraging transplants from healthy donors could save thousands of lives. Second, it troubles me that we often expect the donors of kidneys and blood to act out of sheer altruism, but we have no such expectation of any of the other participants in an organ transplant, like the health care providers or the hospital.