Black market is a term for an illegal economy that is not regulated. Many goods are sold in this manner, such as weapons and drugs.
The Ferengi control the black market for meat on Vulcan. (ST reference: Federation Passport)
The black market thrived on Terok Nor during the Occupation and was run by individuals such as Quark and Ibudan. (DS9 episodes: "Necessary Evil", "A Man Alone")
Quark would later maintain links with the black market when the station became Deep Space 9, although under Odo's constant watch. (DS9 episode: "The Passenger")
Romulan remat detonators are sold on the black market. (DS9 episode: "The Darkness and the Light")
After she stole the USS Platte, Pak Dorren planned to sell its tritium on the black market. (DS9 novel: Time's Enemy)
In 2372, Varis Sul dabbled in the black market for much needed supplies. but almost lost her position as Tetrarch as a result. (DS9 novel: Wrath of the Prophets)