While deciding what to play, I'd inform them that if we play DS that there is slavery in the setting but there will be no permission for player characters to participate in that system. No taking slaves, no buying slaves, no taking hostages and selling them into slavery. Nada, zip, zilch, zero. It exists for flavor and for possible story hooks that may bring the PCs into conflict with those who condone and practice it.
Edit: Short Version - Something existing in a setting is not the same as saying PCs can use it.