Buy and Sell Rates
All vendors and shopkeepers have a percentage rate that represents the markup they sell goods over the item's face value. They also pay a some fraction of the fave value when you sell items to them. For example if you buy a Chainmail Basinet from a shopkeeper with a sell rate of 125%, 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.
For this reason players seek out shopkeepers that buy at a high rate and sell at a low rate in earn/save the most pyreals. Early in the game's history there were a small number of vendors that sold below face value or bought above face value. This made it possible to develop trade routes where you could make money by buying supplies at one vendor then traveling to another vendor to sell them again. At that time pyreals had burden and took up more space than they do now which limited the amount to be made per trade run. However Turbine has since changed vendors to never sell below face value and never buy over face value, which means it is no longer possible to earn money in this way.
A small number of vendors utilize special forms of Currency and for this reason sell at 100% and don't buy anything. For example the Colosseum Coin Collector values all its items in terms of Colosseum Coins.
This is a list of links to the vendors for each particular rate.