You will see a change--when you get the character with 4 bartering to talk, ALL characters in the trade panel (if you switch between them) get the better prices. That is, merchants offer wares cheaper, and you sell higher.
Mouse over any item in the shopkeeper's stock, and something from your own inventory and note the 2 prices shown for buying/selling. Exit out of the trade window; select another character with lower or zero Bartering to initiate the dialog and enter the trade screen. You should see poorer deals.
