I agree that in most markets what you said would be true, however the organ market is a bit different than most other markets. Since organ transplants are so controlled, the black market is actually pretty small in the US. You need to have a hospital to perform the insertion surgery, and there is only one distribution point, so it is fairly easy to know where the organs come from. Currently no hospital could pretend not to know where the organ came from, so it is very difficult to have doctors or hospitals willing to use black market organs. This keeps the market extremely small.

If you open up the distribution, this would change. It would create the possibility of denial for hospitals and doctors, and this is what would spur the black market in organs.