Each point of Bartering is equivalent to 20 points of Attitude (4 points of Charisma). A rate of 100% can be achieved with 5 bartering and 100 attitude, which would allow you to buy for 170% of an item's value and sell for roughly 58.8% of an item's value. This rate can be improved by raising Bartering past 5. It would require 100 Attitude and 12 Bartering to buy and sell items at the same rate.