This is something I was looking at ages ago, and had a tiny bit of success with, but 1) I cannot recall precisely how I did it, and 2) whatever I did affected ALL objects the Player wanted to buy/sell.
What I'd like to do is make an item which, regardless of the Player's Speech Skill, and regardless of which Merchant they take it to, will always sell/buy for its exact value. So if I value it at 50, it will always sell back and forth for 50 gold. There is a "Modify Buy Price" and a "Modify Sell Price" in the CK, but I seem to recall they do not work precisely the way intended...
Does anyone have any thoughts/ideas?