I was there once and I believe it was a market area, people selling things. Sort of a flea market for lack of a better description. Would that be correct or am I thinking of something else?

This is a museum about slavery. You are thinking of the Charleston City Market

The market is where people sell things and slaves also sold things at that market. The slave market is where slaves were sold it's two different locations. The slave-market is a museum.

This is the market where the slaves were sold.

definitely not The City Market. Do they every hold events there? Maybe I ws in a different area.