In a large way, however, getting a fair price is what the controlled market is for. Fairness doesn't really exist, when a CMDR is asking some shady contact to please take some stuff off their hands. The shady buyer isn't a charity. And it's tough that CMDR is in a hard spot. But the black market buyer is looking for a profit, too. A CMDR who's in a pinch is easy to take advantage of, and the black market dealer's offer is really a "take it or leave it" situation.
Of course, I'm writing this from the perspective of imagining a quite vivid, living sense of the Elite galaxy, of dealing with legal burdens, of financial and/or situational woe. A black market isn't a fair or sanctioned place, really. Sometimes, advantages are going to be taken of those who are in a bad spot.
EDIT: I'm imagining here looking at an NPC offering me 50% of my cargo value and thinking... "Sorry piece of garbage." Then maybe having to take the offer, anyway. That's a sort of interesting layer of interaction that might crop up, by letting things get a bit "unfair" on this front.
I'm thinking of two scenarios where for example I need to sell otherwise legal/non-stolen goods on a Black Market?
I'm pirating at a CG, threatened a CMDR than they've abandoned 10t of cargo so it's not stolen. I turn up to the station, and as I have a bounty I can't sell the Gold on the Commodity Market. Instead, I'd want to duck into the Black Market and sell it for a reasonable price there.
I'm delivering some passengers to a station where I have a find/bounty. I complete the first lot, and they give me some cargo, which fills up my cargo space. I can't complete the second passenger mission as they are also giving me cargo. I can't sell the cargo on the COmmodity Market, so have to dump it? Really? Surely better if I can simply duck into the Black Market and sell it at least for something there?
And I suspect there's a myriad more scenarios where it would just be nice to be able to sell non-stolen/non-illegal goods on the Black Market when you don't have access to the station's Commodity Market.