At some point in the campaign, a stall on the right side of the trade port, opposite Molly and Holly will open, with random traders dropping by. I suppose they are from the resistance or civilians and can be considered "black market".

Some exploration missions require you to retrieve a specific item before you can exit, sometimes it's a large case, sometime it's some sort of barrel, and one of the random traders will trade these for 200 cash, ?? research points, or 3 efficiency points.

Oh, those two traders are considered the black market? Alright, I'll try and keep an eye out for any of them making trades for efficiency points. Haven't noticed it before. Thanks!