Could you give the exact quote for the bit you're talking about? If she indeed almost gets sold as a slave, then there's nothing for me to explain; Quark simply lied or was incorrect when he said the Ferengi don't practice slavery (the most reasonable explanation being inconsistency on the part of the writers).
Also, you're repeating multiple times that females in Ferengi society are slaves - I already said that I don't dispute this, so you're preaching to the choir. The word "slave" is quite broad and can be used, at least in English, to denote all kinds of "unfree" or "less free" people, and Ferengi women are definitely lacking in freedom. My point, however, was that Quark was almost certainly referring to a slave in the sense of a class of people who do involuntary labor and whose status is hereditary; this is what people generally mean by "slave", and the fact that the writers did not specify this or use a more precise term like "chattel slave" was because most people already understand the word "slave" when otherwise unqualified to mean a chattel slave in most modern contexts. Ferengi women are actually forbidden from engaging in labor, which is an injustice of a different kind. Quark being a Ferengi businessman, it's quite logical within the story as well for him to refer to a slave in the economic sense rather than in terms of rights or freedoms (again, I repeat for the final time that Ferengi women have little to no freedoms, that people with as few freedoms as they do can indeed be called slaves according to the Oxford English dictionary, and that their status is unjust).
It makes much more sense to assume that Quark was talking about chattel slavery, rather than to assume that he was lying about the status of Ferengi women, since he's OPEN about the fact that Ferengi women are oppressed in various episodes and indeed champions the idea that they should be oppressed until he learns his lesson. I almost immediately understood Quark's comment about human slavery to be a reference to the common human practice of chattel slavery, not an assertion that Ferengi society is free of oppression; I'm sure that most other viewers did as well. If you told someone who'd never watched Star Trek that all Ferengi women are slaves, they would probably get the wrong impression of how their society functions; that's because "slave" is or has become a very broad word, but not all types of unfreedom are equal to chattel slavery and chattel slavery is the first thing that comes to mind when someone says "slave".
It seems almost certain that that was the writers' assumption as well, the only possible alternative being that this is a plothole since Quark has no motive to lie about how Ferengi women are treated.