I've read through various studies listing the professions of slaves in South Carolina, and "prostitute" doesn't appear on any of them. (Take that for what it's worth, there's any number of reasons why it's not conclusive.)
Sex with slaves did happen quite frequently, but it wasn't something that met with public approval in South Carolina. The richest black man (a freed slave by the name of Ellison) in antebellum SC was very probably the son of a rich planter and a house slave. He was raised as a slave, but was given an education and eventually freed by his father when he came of age. But he was never publicly acknowledged as the planter's son due to the cultural mores at the time.
Yes. There was trade in slave women acquired for their sexual desirability. It was a varied market, but a particular demand was for light skinned slaves born of mixed unions, as white as possible. The prices of these "almost white" slave women could go for considerably higher than the acquisition cost of strong, able bodied black men.
Further reading:
Edward Baptist, "Cuffy," "Fancy Maids," and "One-Eyed Men": Rape, Commodification, and the Domestic Slave Trade in the United States at http://www.jstor.org/stable/2692741 has an extensive discussion.
and
Negroes for Sale: The Slave Trade in Antebellum Kentucky also takes up the local trade in these "fancy maids" as they were called.
Do you mean using them as prostitutes for themselves or "pimping" them out to others?
If its the former, this thread contains a lot of information