The item has a "Value" not a price. The value is the general value of the thing. Weapons are more valuable than a Skull.
But what a shop keeper is willing to pay for an item depends on how well he likes you.
As mentioned before, the game does have an "appreciation" score, if you hit an NPC with your weapon you can see this score drop.

Most vendors will like you more if you get them a good deal, and this give you good deals later on.

I would love for an option for the party to be able to negotiate for better prices though. Either before hand, or after the deal, using a charisma check