Enter that master of quick and easy short-term solutions with long-term side effects: the thief.
Thieves are adept at locating merchants who might be overlooked by more scrupulous characters, the sort of merchants who deal in dubious goods with an exclusive clientele. A thief in search of a buyer or seller of a particular sort of item may make one Hear Noises throw per day spent in seedy taverns. On a failure by 10 or more, or on a natural 1, there are simply no goods of that kind for sale on the black market this month. On a success, the thief locates a merchant who allows him buy or sell the good as if the market class was one class better (like Commissioning). There might be a catch, though.
On 1d6 for buying, or 1d4+2 for selling:
1: The goods are hot. If purchased, enqueue trouble with an XP value about four times the value of the goods. A successful Finding Traps roll lets the thief read the situation and choose to bail on the deal, or demand a discount of 1d6*10%.
2: The goods are defective. A Finding Traps roll lets the thief sense that something is up and choose to bail, or demand a discount of 1d6*10%. Defective magic items are cursed. Defective arms and armor get a roll on the Scavenging Treasure tables on page 209. Defective animals roll HP twice and take the worse. Etc.
3: "I'm gonna need a favor." Merchant needs a quest done, of magnitude proportional to the value of the goods, before he is willing to sell. A reaction throw of 9+ can make him accept a 1d4*10% markup (or discount if the player is selling) instead.
4: "But you owe me one." Merchant wants a promise of a favor that he can call in later before selling. A reaction throw of 9+ can make him accept a 1d4*10% markup (or discount if the player is selling) instead. If owing a favor is accepted, throw it on the trouble queue.
5: "I'm feeling lucky." Merchant would rather play a game of chance, with the goods as one party's stake and something of equivalent value as the other party's stake. Reaction roll of 9+ to convince them to just transact, or make opposed d20 rolls (+4 for Gambling proficiency, +4 for a successful Picking Pockets throw for sleight of hand; natural 1 or by 10 or more on the Picking Pockets reveals you were trying to cheat and triggers a forfeit), winner takes all.
6: No problems, a pleasure doing business with you.
If PCs attempt to pass off hot or defective goods, the merchant likewise makes a Finding Traps roll and may demand a discount or refuse to transact if successful.
I guess you could let any class do this, with the standard 18+ Hear Noises and Find Traps throws...
Bonus, 1d8 black market merchants.
- Choo-oock the Bugbear Alchemist. Lives in a wagon drawn by a giant beetle, wears a crumpled and stinking wizard's hat. Potions smell like feet and vodka, guaranteed "to put hair on you face. Money-back guarantee? What? No, just regular guarantee." Deals in poisons too, which smell suspiciously similar. Skips town frequently.
- Guillaum de Crochefontaine, dissolute noble scion. Enjoys a wide variety of questionable substances, gambling, and sleeping with married women, gets away with it on account of family connections and being a pretty decent duelist (Fighter 3). Magic sword enthusiast.
- Lazy-Eye Lud, watchman on the take. Pudgy, mid-thirties, five kids to feed, doesn't actually have a lazy eye. Sells confiscated goods, sometimes steals from the armory. Might shake you down if you get caught and jailed though.
- Owen One-Foot, retired whaler. Skin like leather, heavy wool clothes, vicious grin, missing a foot (crushed between boats, then amputated). The ship he worked on, the Bloody Mary, occasionally does a spot of piracy when the whaling isn't so good, might be able to hook you up with some bulk cargo, slaves, or a prize ship. Or fifty stone of stinking whalemeat, if you need dragon bait.
- Mistress Ludhevna, proprietress of the Slap and Tickle. "Our clients occasionally depart in haste, when their wives arrive, and neglect their personal effects. Could I interest you in some chainmail? Perhaps this fine dirk? Yes, yes, I'm sure you already have a fine dirk, save it for the girls, dear."
- Herr Gunther Grosse, gourmand and purveyor of exotic meats and livestock. Fat, bald, moustachio'd minor nobleman, has a warehouse by the docks and a villa outside of town. Has fingers in many pies and friends in high places. Throws crazy parties, seems to have repeated bad luck with young wives dying during childbirth. Rumored to be a cannibal.
- Three hundred rats in a filthy robe, formerly known as Geirmund the Magnificent. Used to be a wizard, but flubbed a magic experimentation roll, can't do the somatic components anymore. Can, however, explore warehouses and steal keys. Knows a thing or two about magic items and curses. Communicates in a cacophony of high-pitched, squeaky voices. Hates cats. Shits everywhere.
- Big Hilda, taverness and pit fight organizer. In her 40s, thick-set and ill-tempered. Yells a lot, handy with a cleaver. Runs the Rusty Nail, a bad tavern in a bad part of town, with a fighting pit in the cellar. Has a bunch of kids who cook, serve, clean the pit, keep book, etc. Husband died under mysterious circumstances. Deals in arms and armor of dubious quality, slaves, animals, meat, mercenaries, and stimulants.
Man I'm gonna name everything thief-related after cocktails now, that was a good idea.