Duke's "Black Market" doesn't feel like one, as he is selling ship parts, accessories and outfits previously unobtainable without commendations for doubloons inside the tavern in front of the whole outpost. He is also part of the "Bilge Rats", but we only ever see him, so it feels like they don't even exist.
My suggestion is to leave Duke at the tavern so you can turn in loot and buy missions and commendation letters from him, but create a new store with different Bilge Rat members on the back of outposts, like makeshift shady stores that look like some illegal system away from the other factions that sell all the monthly cosmetics.
This will make his faction more alive and fleshed out while also exploring all those "back entrances" and different piers on the Outposts.
