Well the barter formulas are
Buying price = (Item Value * fBarterBuyBase) + (Item Value * Skill * fBarterBuyMult / 100)
Selling price = (Item Value * fBarterSellBase) + (Item Value * Skill * fBarterSellMult / 100)
The fBarterBuyBase, Mult, and their counterparts can be adjusted via the gameplay, settings tab under geck. I really don't know of any other way to adjust the actual skill effects. I see a fAVDBarterBase, which could translate to something like fActorValueDBarterBase or something, hopefully someone can tell you more.