A slave market was an establishment or event where slaves were sold.
In 2151, while contemplating what loot to take from Enterprise NX-01, Krem suggested he and his fellow Ferengi take the female crewmembers, adding, "If we leave now we can make the next slave market on Stameris. Jonathan Archer, who had told them to take what they want and leave, changed his mind upon hearing this. (ENT: "Acquisition")
In 2153, T'Pol and Pierce found themselves at an Orion slave market, waiting to be processed for the upcoming auction. (ENT: "Bounty")