Charisma
Charisma is the overall attractiveness and persuasive ability of your
character. Characters with a high charisma have an easier time bargining
with merchants, as well as convincing monsters not to attack them.
Not suprisingly, any character considering becoming a merchant should
have a high charisma.
(Note that charisma only affects actions of NPC's and has nothing
to do with the reactions of players.)