Actually, your system, if we run with "highest charisma" or total party charisma can exploited on the very first visit to the merchant.
Only because you don't really understand what's being suggested, frankly.
The problem is that I understand it all too well. Your easily exploited system isn't any better than what we have now, and, in fact, could be worse. I mean, it's not like anyone's going to roll up a bard, sorc or warlock with max charisma, right? Wait, of course they will. Surely nobody would then take Wyll with them, to get that extra charisma boost for better prices, right? Wait, of course they will. At least, under the current system, we have to give something to get something, instead of just taking really high primary stats on a charisma based character, and getting discounts right out of the gate, on top of being able to just sell all the stuff we might have bartered to obtain those discounts. Wait, that doesn't do anything to prevent an influx of gold into the game, while, on the other hand, bartering items and gold off for favor will. Not that it's going to make a lot of difference either way, as I said, games that don't have this kind of system still wind up with extremely rich characters. So your system will not only not solve this "problem", it will, instead, exacerbate it, because items that may have been bartered off will instead just be sold, adding even more gold to the game.
