So, like group approval raising from repeat business?
The specific value to use is a bit beyond the point. It could be based on your overall reputation as heroes, your relationship with the faction, your AVERAGE charisma value as a basic standard, eventually it could take into account a little discount on top basic prices because you did a quest for the trader, etc.
Eventually you could get even temporary bonuses on top, like using Charm and Friend, that come with short-term benefits at a long-term price (your reputation with the merchant will plummet after he's freed from the enchantment). But this mostly is already in place.
The point is: it should be mostly a background thing and not something you should be able manipulate so easily with so little effort.
And it should be party-based, simply because otherwise it's going to actively encourage all that annoying juggling of items from one inventory to the other.
Have you ever realized that if you switch to a different companion while trading prices can change significantly? While this could make somewhat sense in abstract, what it does is making the tabbing between companions non advisable. The most efficient strategy when dealing with a trader is to open the group inventory, pack your "charismatic leader" like a mule and sell or buy everything with him.
Not hard to achieve by any mean, but time-consuming, annoying and very "meta-gamey".
