I thought to myself- "How come the nasty Duergar merchant sells his goods to one of their races most hated enemies, shield dwarves, for the same price as to other Duergar?"
I think a different markup price for goods based on your race and relation to the merchant would be cool. Substantially higher prices for kobolds / goblins at nearly every merchant, for example, and higher prices for dwarves buying from Duergar merchants. I'm sure there are more possibilities others can come up with as well!