Also, you can take out gold you would get from selling or give more when buying to increase vendor favor. I did this for first two vendor's since its just them for a while.
Yeah ,this was in EA already, but it was nerfed considerably since Patch 9 (it used to be that 150 gold were enough to max attitude with anyone, while now it requires almost 20X as much).
The Charisma thing on the other hand wasn't a thing back then.
What's worse, it seems that its impact is MASSIVE, too.