I think it would be more realistic if there were varied prices on the slaves you sell. yes it may seem they all go to the same place.(salt mines) but i for one feel that there would be some ransoming to family members or nobals for sons and or daughters in the army, there by making it more profitable to by them from you. and of course if your slaves are pesants they may not be as hearty and probably not worth as much.
1. pesants refugees and such worth maybe 10 to 50 denars
2. farmers (they work hard any way) and low lvl troops 50 to 100 denars
3. mid lvl troops 100 to 200 denars
4. high end troops 200 denars
5. the nobles (sell them out from under your king or count

) 1000 to 5000 denars randomized with a 10% chance to get 0 and a visit from the opposing war party!!!
any way its just an idea