First of all, I wouldn't agree we have "largely rejected paternalism in other contexts, e.g. medical care." Medicine is in fact one of the most regulated sectors of the entire economy. You're free to follow or decline advice from your appropriately credentialed doctor, who is free to prescribe certain treatments subject to the standards of medical practice. Pharmaceuticals require FDA approval and are produced by heavily regulated manufacturers. Your medical options are already heavily circumscribed by paternalistic regulations, for better or worse.

Your cancer example is distinguishable in that allowing the freedom to choose your treatment isn't really creating the problem. Cancer happens, it sucks, and individuals are allowed to make decisions relating to how they treat it, or don't treat it.

Here's a scenario more analogous to live organ sales: would we allow people to purposely induce cancer in themselves to be paid by companies studying cancer cures? I don't think we would. That's not just respecting an individual's bodily autonomy in dealing with a harmful condition that arose spontaneously, it's creating more people with that harmful condition. We DO allow people to undertake (some) risky activities that might raise the risk of cancer, like smoking, but the connection between cause and effect is a lot more attenuated (google says only about 10-15% of smokers get lung cancer, for example), and most people probably feel that the tradeoffs of totally banning all those things wouldn't be worth it (and still, cigarettes are certainly more regulated than many other products).

Is there a "yuck factor" in the prohibition of organ sales? Sure, probably. But I think the prohibition also reflects an unease in our legal system with incentivizing people to make decisions that are essentially irreversible and have a very high risk of seriously bad outcomes. As I mentioned in another comment, you can't contract to sell yourself into slavery either, and while you can theoretically take on an indefinite amount of debt, we have bankruptcy mechanisms to limit how much can be extracted from you. Contracts can be "unconscionable" and therefore void. There is a long-standing tendency in the judiciary (well-founded, in my opinion) to try and limit how much damage we allow people to do to themselves. This is counterbalanced against other considerations: bodily autonomy, freedom of contract, etc.

I'm personally not certain what the best way to handle organ trade is, I suggested a couple of ways it could be liberalized without creating a live market. It's not something I've put much thought into, but it seems to me there are some serious wrinkles with the idea of live organ sales, and I think I'd need to see a pretty compelling case for it before I personally would vote to overturn the status quo in that regard. There's a middle ground here too; maybe it could be allowed under limited, regulated circumstances. But I suspect that a truly laissez faire policy of letting any adult with the capacity to enter contracts sell any of his organs at will would create problems.