"Barter" is a passive skill in FOnline. It determines the sell value of your items and the buy value of the items you want to buy from an NPC. In other words the higher your Barter skill is the more money you can get for an item, and also the items sold by the NPCs will become cheaper for you to buy.
The Barter skill is influenced by the S.P.E.C.I.A.L. stat Charisma
1x Charisma = 4x bonus to Barter
The starting skill level of Barter is calculated by this formula:
4x Charisma + 20% if tagged