There are a couple examples of Ferengi utilizing what is essentially slave labor but creating the illusion that their 'slaves' are earning money. One example is the VOY episode 'False Profits', another I can think of (non-canon) is the TNG novel 'Debtor's Planet'. It seems the difference between Ferengi slavery and human slavery is that in Ferengi slavery, the slaves earn money, but they have to pay it back in somehow, and use every bit they earn to 'purchase' the basic necessities of life that in human slavery would be considered an expenditure by the slave master. It's just a cultural difference, Ferengi are so capitalism-based that they even impose this part of their culture on their indentured servants. So in a sense, Quark is right - it's not slavery in the minds of the Ferengi. To have true slaves that didn't even have money to spend on their own basic needs would be an affront to Ferengi culture, as that would ignore the opportunity to buy the materials the slaves need to survive, and sell the materials as a profit to get back the wages they are paying. i.e. say a Ferengi pays a worker 5 strips of latinum a week. They purchase the food a worker would need to barely survive for two strips of latinum, and then sell it back for five strips, making a profit of three strips in the process, since these workers have no other options.
(ahem...sound a little like life in the American lower-middle class, anyone?)
(ahem...sound a little like life in the American lower-middle class, anyone?)