The bartering of the character that instantiated the trade will be taken into account, not the bartering of the current trade inventory character.
See it this way, Roderick can be trading with a trader in Cyseal while you let Scarlet venture forth.
You can switch to her inventory to sell items that she possesses, but it's still Roderick doing all the talking. Even if all party members are near the trader, it's still Roderick trying to sell an item that Scarlet happens to have in her inventory.
For each point of bartering, you get around 20% discount or boost (bartering 5 makes you pay half the buying price and doubles the sell price compared to a bartering 0 character)
Attitude is also taken into account though...
note that these discounts also apply to the repair and identify prices!
