Barter "Trading and trade-related tasks. The ability to get better prices for items you sell, and lower prices for items you buy."
The higher this skill is, the closer the price will be to its "real" value. (not more than 90%)
Soft caps at 125% (not true for Atoll - 150% and GIZG - 300% for non-citizens).
Commonly, the farther south and east the city is, the more Barter skill you will have to have. Also the time you can be in a "barter mode" is limited, and depends entirely on your Barter skill.
0%+(4*CH) is the % your character will start with.