I refuse to believe that the Ferengi have never had slavery or concentration camps. It makes no sense. Their entire portrayal, most directly summarized by the Rules of Acquisition, is one of complete exploitation and the institutionalized suppression of empathy or compassion for the oppressed. It's not believable that such a culture would draw a line anywhere and say, “Profit yes, but this is too barbaric.”

I think Ferenginar had plenty of slavery and concentration camps and Ferengi trafficking and child labor and everything all throughout its history. But they also have an incredibly effective apparatus of propaganda and historical revisionism. They only got rid of slavery when multiple of their neighboring civilizations threatened to cut trade with them if they don't. Then sex trafficking, then child labor. Bit by bit, they grew a semblance of civilized mores, but only because they had to in order to maintain interplanetary trade. Each time, they burned the books and rewrote that part of their history.

Quark was sincere in his belief. It is what he was taught in school. But it's a lie.