All vendors give you the same som of gold for an item (for example Diamond: 180 gold), but buying goods can differences because mostly if you Mindread a vendor he/she lowered their prizes. OR after you do a quest for a vendor. If a vendor lowered his prizes twice, you can buy at half price
(Locke in Black Boar Inn for example, if you choose his side by a quest + mindreading = 2 x lowered)
