After Harribell's explanation, Yang Ling noticed that there was a deep, cross-shaped brand on the forehead of every craftsman, maid, and warrior. According to the conventions of the Tailun Continent, this cross-shaped brand was the symbol for a slave.
Slaves came from many sources, but most of them were captives of war. After every large scale war, there would be large numbers of slaves available on the market. The maids and warriors Harribell was giving Yang Ling were basically prisoners of war or commoners from the losing country.