Is human head transplant research unethical?

What are the ethical arguments against research in head transplants being carried out by Dr. Xiaoping Ren in China?

 
There are ethical considerations based on the objections of animal rights activists. I don't discount them, but I'm not well-versed in their terminology.

The ethical considerations as extended to humans are most obvious to me. Even if such a thing as a head transplant were possible, which is extremely unlikely given the complexity of reattaching a severed spinal cord, what are the criteria for body donors?

 
Here is the ethically challenging scenario: You have two people, 

• one with a paralyzed body but a healthy head 
• the other with a healthy body and a healthy head. 

This is different than organ donation, which is highly regulated and can be done without killing the donor, or with donors who are deceased already. It would be much less feasible to find brain-dead donors with healthy bodies to donate to people with healthy heads and diseased bodies. 

It gets very gruesome, very fast, describing these scenarios. See too Bruce Hall's answer to If Dr. Sergio Canavero head transplant is a success, how big of a breakthrough is this?

Suppose the paralyzed person wants a new healthy body and has a lot of money or power.  What is to prevent the paralyzed person from having the healthy person killed, in order to get their healthy body? (Or would they switch bodies, and the formerly healthy person be stuck with a paralyzed body, while the rich person got a nice new body? Why bother? Would it even be possible to reattach the paralyzed dysfunctional body to the other head?  Probably not.)

It should be obvious that a free market is not the right way to determine who gets to live and who is chosen to "be donated".