Adopting the "opt out" system for organ donation: if a person who would "opt out", then it would be relegated to the end of the lists of transplants, making it (if waiting lists continued to exist) impossible for the opt outs to get transplants (that seems fair!). Everybody else would be an automatic donor, there could also be a special "opt in" feature where if you felt altruistic you could make your donation non discriminatory to opt outs. With the appropriate rules it would work very well with a fantastic amount of donors.
The market solution for vital organs is very week, lots of distortion and waist.

As for live donors of non essential organs, well I think a market could be created... something for society to discuss.

Men are already paid for donating sperm and women for donating eggs (in the US); going to kidneys and livers is just a matter of degree.

"just a matter of degree", dan hasn't an idea what he is talking about, but that's understandable. From Judge Posner's calling torture "being rough" it's a small further step towards opening a market for kids. And so on. Messrs Becker and Posner can join hands with Globke.

Posner suggests that giving organs first to organ donors has a drawback -- people who need transplants will sign up but not be able to donate their organs.

But many people who need an organ have other organs that they can donate. For example, a person who needs a kidney may have a perfectly good heart.

LifeSharers, the network of organ donors who offer their organs first to other registered organ donors, does not screen donors before giving them priority. Anyone can join, and membership is free, at www.lifesharers.org. Fewer than 1% of LifeSharers members need transplants.

Egg donation is a painful process with considerable risk and long recovery period for the donor, so one would think that the 'human egg market' would be very similar to a market for nonessential human organs.

I would also like to point out that donors are paid for blood plasma.

It is worth considering that if transplants were done more, their price would be less. It is true that while you are not compensated when you donate your organ, that is the last time in changes hands for free. Although it has been said that hospitals don't profit from the sale of organs, they can't be disintermediated from from a rich value chain in which the organ itself is certainly a cost of goods. For insurance companies, the rarity and expense of transplants make them among the less easy to underwrite proceedures, distort actuarial tables and drive premiums. Is it possible that some participants benefit when transplants are infrequent and expensive?

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Regarding this portion of your post:
"A physician comments that one problem with a market solution is that it will result in some substitution of the organs of living persons for the organs of cadavers, and the operation poses some risk to the (living) donor and also deprives him of an organ that he may need in the future. So this is an argument for opt in."

Your argument had been that the number of organs available would not change in an opt-out system, because as more cadaver organs are made available fewer living donors will be willing to donate. My comment earlier (I am also a physician who follows your blog regularly) was that, despite the numbers available, this would be a positive scenario because it would reduce the morbidity of organ donation if the ratio of cadaver-to-living donors increased (less risk to living donors). Therefore, it seems this would be an argument in favor of an opt-out system. However, I agree that in a market system where organs can be sold it will increase the number of living donor organs - however, they may not sell for as much if more cadaver organs are available.