There are hundreds, if not thousands, of different cultures and species, all with their differing views. Historically speaking, slavery as a violation of human (and non-human, in our case) rights is a relatively new and vogue concept made common within the past 200 years (and still not accepted across the world). Slaves were a convenient source of manual labour which served as a silent and often necessary backbone of civilization growth and development. Erde does have slaves and an active slave trade, which Verre Zon participates in to some degree.
It is not impossible for a basic republic like Verre Zon to have slaves who are unrepresented in a government setting as more than property. Ironically the two ideas do not clash, since the base assumption is that a slave simply doesn't have rights to represent, instead being the property/responsibility of his or her respective owner.
The Slave Trade itself
There is both a straight slave trade (like what was going on in our world in the 15-1600s) and what we would consider indentured servants - people who work for an allotted time towards personal freedom. Because of the high culture mix, both are used, and players may decide which they would like to see/address in their own games. This allows both extremes to exist, giving people a more open variety of the subject matter. This way a village can be run as a small feudal microcosm or a Southern slave plantation and neither is against the overall setting scenario.
Of course, slaves/slaveowners are playable. But please don't create a character built solely upon fetishes. This is not a Gorean setting.
Slaves' interests in the Conclave
Slaves' interests are not directly represented in the Conclave, as they are property. However there may be Conclave members who represent the interests of those who own slaves.
Slavery outside Verre Zon
There was certainly an official slave trade in Theros, complete with caste laws (which Verre Zon would have borrowed) and both merchants and mercenaries have made their fortunes on that slave trade.
The government would also use slavery as punishment for significant crimes. Verre Zon has no concept of imprisonment (beyond temporary holding cells) nor government-sponsored asylums. Instead, the Conclave keeps a public workforce of slaves rather than have idle mouths to feed. If a criminal is too dangerous or difficult to be a slave then he is killed without fuss or fanfare.
Pockets of feudalism would exist as well. A warlord with the might of arms to force the masses into serfdom will be tolerated if his allegience is with the Conclave.
There will also be small independant kingdoms outside Verre Zon, but near enough to be relevent. Nomads would take slaves from raids. Robber barons who use their position to levy unauthorised tolls on travelling merchants would be in places where the only law is the power of their armed followers.
