The black market argument applies to everything illegal. Of course that risk exists, but the number of those undertaking the activity will be smaller, as you concede. Lack of legal safeguards and medical accountability, and the probability that only badly qualified doctors will offer illegal operations will deter almost everyone from risking black market surgery. Fewer operations must be desirable if it is agreed that the activity concerned should be banned. Black market activity will be vigorously policed – and after all, its usually pretty obvious if someone has had surgery.