A slaver is an individual who is in the black market or otherwise business of enslaving other human beings for trade of profit to be used as unpaid and cruelly treated laborers. The workers in the factory settlement of canton on Higgins' Moon were technically indentured, but, as they were paid next to nothing, they lived the lives of slaves.
Malcolm Reynolds successfully robbed the slaver Wright who bragged about his profits a bar in Santo, and then, with Jayne Cobb, fought with the man and his comrades before fleeing.
Presumably Badger is a slaver as he is seen inspecting a girl in the pilot episode.