I think you're just bitter because Cha and Int don't do anything for some characters, so lots of people are using them as dump stats.
Yes. Yes I am.
Also, isn't it a bit unfair to penalize the whole party for the whole party's average charisma? Shouldn't it be more like the average of whoever's doing the bartering and whoever they're bartering on behalf of, or something?
It's not the average.
Anyway, maybe. But ask yourself this: If you see ten guys walk into a bar, 8 of them look like shit, the other two look really good, and one of the good looking persons walks up to you and tries to sell something on behalf of the other good looking person, wouldn't you be kinda... I don't know, bothered by all the shit-faces even if the thing is coming from the nicer looking ones? I mean, you know they're part of the same group, unless you're planning to get to know them, I'm pretty sure you'd judge them on behalf of their group.
Also, that has also got a problem: you could just have a sort of network of players, giving their items to the second-best looking and having the best looking sell them.
And, I know this has been an issue for a long time, but I just feel the need to point out that skills whose attribute is [X+Y]/2 increase at half the rate of [X] skills. Which I guess is more like 3/4s or higher because of actual skill gain, but still. I'm not sure the flexibility of being able to choose which secondary benefits you want justifies that.
Problem is that the other two alternatives are picking one attribute, and having people complain that it's the wrong one, or having both at normal rate, and having them advance much faster. I picked the lesser, easier of the three that will cause less complaints.
But yeah, the flexibility partially ofsets the halved bonuses.