The game seems to frequently generate quests that you can complete, for profit, by "flipping" slaves or items.  This happens when a delivery quest can be satisfied by a slave or item currently up for sale at the market, and the reward from the quest is greater than the market price.

I feel kind of dirty taking quests like this, and the irony of saying that about a porn game is not lost on me.

Honestly...that's something people do in real life too. They buy something cheaper or in bulk, only to sell it at a higher price on Amazon or something.

And considering that there is no Internet in this world... those who walk around and look for an opportunity should be able to sometimes make easy money.

I personally like this mechanic so it will likely stay

My head-canon reasoning for this is that as a landowner in game you have access to extra merchandise that the guilds do not. So it is more like the guilds are paying you for your influence to obtain the items they want.