A demigod is a mortal being infused with divine energy. This essence comes from a divine spark in their soul that activates due to some external traumatic event. This spark is very rare, and most people with them go through life never realizing their potential. These gods are super-human in a way similar to Captain America, and are smarter, stronger, longer-lived, and tougher than regular mortals.
On the rare occasion that a god dies, through murder, war, etc., their soul departs to the afterlife. However, their bodies are incapable of rotting, and remain in perfect condition. Their organs and genetic material are still super-charged and continue to remain viable. Because of this, there is an black market that trades in these organs and other body parts. An individual can use these parts to replace their own, gaining the benefits of those supercharged organs, such as kidneys, liver, eyes, etc. A human will never be able to achieve the full benefits of these transplanted parts, due to the divine essence being dissipated. Nevertheless, they will be leagues ahead of any other mortal.
Ignoring law enforcement and cost, I need a reason for why the sale of these organs in the underworld isn't more widespread. A god's superhuman body parts and organs are fully compatible with other mortals. Anyone with the money to pay for these illegal transplants would be able to supercharge their own bodies with them. While it has been outlawed, there will always be a black market for them. Despite the benefits of their use, they are not widely preferred compared to mortal organs. There is a significant failure rate to the process, even though most transplants are successful. Why would this be the case?