De facto slavery exists for women in Ferengi society, but the fact that no other apparent slave caste exists along with how quickly Ferenginar's patriarchy was restructured, if not quite dismantled, I think one possible barrier to an effective slave industry (certainly chattel slavery) is that there would be no concept of a person or group of people who should not be trying to earn profit, and the power structures that exist would not punish an enslaved person who was able to skim profits from any labor or administrative work they performed, nor would it necessarily reinforce their status as a slave if they were able to buy themselves out of their position.
I think Ferengi probably thinks that a person or business can own another person, but I don't think they can conceive of a person who can't/ or shouldn't own anything. This is like how it's not illegal to attempt to escape prison in some countries - There's a recognition of the desire to be free.
Now, that's all a huge problem when Ferengi develop interstellar travel and encounter full-blown slavers like the Orion Syndicate and other criminal elements, because 1) they'll not only participate in the slave trade for all the reasons you said, but they'll be really good at it, and 2) they won't fully understand the power structures beyond wealth which makes enslavement so egregious.
I wonder: Quark's weapon trading may not have bothered his conscience because of him adopting human ethics, as he feared, but because he was parsing his own ethics with a fuller understanding of the implications of what he was participating in as he was learning more about alien cultures.