in Bg1 and Bg2 the party leader i.e, whichever character is currently in the first [F1] slot is the one used for shopping... even if another character with higher \ lower charisma talks to the shop keeper the leaders charisma is still used
that is the way D&D should work
Luke Skywalker: I don't, I don't believe it. Yoda: That is why you failed.