A black market was a market consisting of all commerce on which applicable taxes and/or regulations of trade were being avoided. Items could be purchased on the black market by those not legally able to purchase them otherwise, often for a substantial fee.
In the 2230s, a Section 31 operative on Qo'noS learned that a time crystal had hit the black market near the Orion outpost. Leland had his assets in the sector steal it. (DIS: "The Red Angel")
Quark admitted that he ran a black market on Terok Nor during the Cardassian Occupation of Bajor. (DS9: "Necessary Evil") Ibudan used to run black market medical supplies through Terok Nor during the same period. (DS9: "A Man Alone")
Constable Odo sometimes prepared for an important operation by leaking word about it to the local black market. In 2369, Odo mentioned to Quark the upcoming shipment of deuridium arriving to Deep Space 9, much to the displeasure of Lieutenant Primmin of Starfleet Security. (DS9: "The Passenger")
Razka Karn was a smuggler, thief and black marketeer with at least twelve outstanding warrants for his arrest on Cardassia as of 2372. (DS9: "Indiscretion")
Remat detonators were illegal explosive devices that could be purchased on the black market. (DS9: "The Darkness and the Light")
The Mari had a black market that dealt explicitly in violent memory engrams (which were illegal in their society). (VOY: "Random Thoughts")
In 2374, the crew of the USS Voyager purchased explosives on the black market to fight against the Hirogen in a holodeck simulation. (VOY: "The Killing Game, Part II")
