There's a secret stat "approval" just like in the Divinity games. You can increase approval by giving people things like money or chests or bones or whatever.
First you barter -> give -> complete trade -> exit diologue then you get the approval +5 or whatever message.
In the divinity games the more someone approves of you the the closer you get to "true" buy and sell value. I haven't tested to be sure it works the same way here, but I assume that it does.