Barter affects both the prices a player pays to a NPC for items and the cash received when the player sells items to a NPC.
The exact formula is unknown but it is based off both the amount of skillpoints you put into the barter (BRT) ability, and the level of your intelligence. This means that spies and monks will always get better prices than gentanks, even if they all put 0 skillpoints into the barter ability.
There is an utility written by some of the members of an old clan from Neocron 1 called NeoBRT which performs a basic calculation based on user testing of specifically the barter skill available from Techhaven.org - but this should be used as nothing more than a guideline, while the calculations for barter haven't changed much over the years this utility doesn't take into consideration the intellect level, which also has an effect on the prices received.