Thanks Baardvark, that really helped.
One thing I'm not sure about is how to make this apply to party members, but not the caster (because the caster doesn't need to buff their own stat, only share it).
It seems like I would need to do something like this:
CharacterApplyStatus(__Me,_BarterStatus,-1,1)
But instead of passing in __Me, pass in each party member. Do you know how I get/reference each party member?