I have about the exact same questions, that was I going to ask at some point in the Help sub-forum. But since you have asked, I'll follow this thread for the answers.
The only thing I know so far :
- Charisma (most likely) determines how good the prices you get are. While inside the trade screen with a trader, if you switch to a different character, the charisma of the newly selected character is taken into account. So a short sword is going to be sold at different prices, during the same trade screen/conversation, depending on whose inventory it is in. Which is very ... hm ... not a great idea.
