A huge part of it is by sheer nature of the black market, traditional dispute resolution is out of the question.

You can't go to a lawyer and say, "This dude backed out on his deal, cheating me out of $10,000 of grass."

So the only method left for many grey/black market areas is physical retaliation.

Outlaw a market, and only outlaws (and their own twisted senses of justice and retribution) will thrive in the market.