Trade in human body parts is mostly forbidden, although donations without compensation or for “coverage of reasonable costs” are allowed. One reason is that trade creates the incentive for criminals to harvest organs against people’s will. In the worst case, a young and healthy person is killed to get all their marketable body parts. Another problem is that stupid people may sell their organs voluntarily and later regret it.
The dangers differ depending on how damaging the removal of the organ is. Trade in hearts encourages killing more than trade in donor blood, although even for blood a victim can be drained completely if the price is high enough. For criminals, the complexity of organ removal and how fast it needs to be delivered to the recipient also matter. It would make sense for the restrictions and punishments to correspond to the danger of organ robbery and the associated damage.
The one tissue type currently transferred between people for which organ robbery and overdonation seem nonissues is sperm. Forcing someone to donate against their will is possible, but causes no permanent damage (in my medically ignorant opinion). Too frequent donations lower the quality (number of cells per unit of volume) in a detectable way, which would make most robbed sperm unmarketable. Yet payment for donor sperm is still forbidden in Australia (Human Tissue Act 1982) and many other countries. This may be a knee-jerk extension of the laws against trade in human organs, or there may be some reason I have missed.