Organ trafficking is an illegal activity that takes place on the black market. People buy and sell their organs to people in need.
One of the popular organs sold on the black market are kidneys. People sell their kidney for thousands of dollars. Many people who sell their kidney are poor and need the money to survive.
Many of the people who buy the organs are in need of a healthy kidney to live. They are desperate for a kidney so they buy one on the black market. The price they pay for this illegal operation is a lot more then the buyer receives. The people who need the organ usually pay over $20,000 for the operation.
The difference between the buyer and the seller in money goes to the doctors performing the transplant and the organ traffickers.
See here for a list of organ and kidney prices on the black market.