Having PCs deal with slave traders is a tried and true trope going back all the way to TSR's classic Dungeons & Dragons module A1 Slave Pits of the Undercity, first published in 1980. (Reissued as part of Scourge of the Slavelords in 1986.)

It makes a bit of a difference when people are creating slaves... and when it evolves organically so that the people involved are blind to the moral questions involved. Now there are some ideas for a setting.

Transcript

[SFX]: < squee be-ting ezang pa-ziku-ting gidip-whroop >
C-3PO: Have you ever seen me throw anything? I'm a translator droid, not a pitcher!
[SFX]: < doop zizz pa-ziku-ting >
[SFX]: < fop-jing doop bleep po-spip-ting >
C-3PO: You better be quick about it.
Leia: If you two are plotting a droid revolution, now's the time.
Luke: Yeah, because that worked out so well for the humans in Sally's campaign.
C-3PO: They were the oppressors, creating slave beings for their own amusement.
Leia: Is it going to be any worse for us than this situation?
[SFX]: < squee bop bippity bedoop ka-beepop ping >
R2-D2: Depends how you feel about droid polish.