Sale of organs in the United States

It is illegal in the United States to pay for a transplant organ, and doctors cannot perform a transplant operation unless the patient is at the top of the official list. However, some wealthy people in need of a kidney transplant go overseas to other countries where the laws do not address the issue. Suppose a wealthy person finds a poor person willing to sell his or her kidney and then purchases the kidney. What do you think of this? If the poor person is willing, is there any harm in it? Why does the United States prohibit the sale of organs when other countries do not?