It all depends on your definition of "successfully". They are successful in a sense that they exist, and that money continues to flow. It doesn't mean that the social effects of such a market are beneficial. Many illegal markets are highly monopolized, often coercively (e.g. "if you try to sell here, we'll shoot you").
Even online illegal markets have coercion. Remember all that assassination stuff around Silk Road?
But I also seriously doubt that most illegal markets are online. Even first world countries have actual physical black market networks for illegal goods. And if you venture into a country where corruption is routine (which is where most of the world lives), illegal markets are literally right around the corner - drugs, fake IDs, firearms, you name it.
(Personal side note: it can be a very... eerie... experience when you happen to run into such a thing by accident, and realize just how close it was all the time.)
And most of those physical markets definitely have coercion aplenty.
