Yes, if a person wishes to sell his or her kidneys or other body parts that they can live without they should be able to do so. In reality the hospital make money from organ transplants, why shouldn't the person donating not make a profit as well. If the removing of the organ does not kill the donor, then they should be allowed to sell their organs.
Anyone can sell anything that belongs to them. No organization or government has a right to interfere with someone's ability to sell, alter or effect their bodies. That body is not the property of popular opinion or the responsibility of outside entities. Any moral or ethical reasons are determined between the person and his/her God or gods. No one else should have a say to it. They can profit from their bodies as they choose without looking for consent.
Yes, people should be allowed to sell their kidneys, because both people think that they would be better off. If someone thinks their life would be better by selling their kidney, by all means, let them do it. A person who is dying will want the kidney and appreciate it very much.
I believe it is far more important to provide organs to those who need them. For this reason, I do not believe there should be a market for organs and people should not be allowed to sell a kidney, if that is what they want to do. People shouldn't receive organs because the can afford them, money shouldn't be a factor in any venue.