If you're refering to the HRP server, there's an NPC near the boathouses you talk to. There can only be two bMD's, though, so the limit could be reached.
If you're actually refering to the Half-Life 2 Roleplay server: You do not become a black market dealer on the HL2RP server. Firstly because we don't have scripted jobs of that sort, and there are not classes and such for it. Secondly, the whole planned Black Market system will mainly be implemented by the administration to get a small flow of guns, ammo, and more rare IC goods into the city system. However, that being said, gun restrictions are sitll rather tight on this server. Only trusted people, with well developed characters, will be able to become 'black market dealers' per say. Not meaning this is a rude way, but you've only recently registered/joined this community. You'll need to get some time here and get a good, involved reputation within the community before you'll even have a chance and getting that kind of privlige.