All vendors and shopkeepers have a percentage rate that represents the markup they sell goods for over the item's face value. They also pay some fraction of the face value when you sell items to them. For example if you buy a Chainmail Basinet from a shopkeeper with a sell rate of 135%, you will pay 585p instead of the face value of 433p. If you were to sell the same coif to a vendor with a buy rate of 95% you would receive 411p.