Buy and Sell Rates
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.
Rates
This is a list of links to the vendors for each particular rate.
Types
A small number of vendors utilize special forms of Currency and for this reason sell at 100% and don't buy items. For example the Colosseum Coin Collector values the items in his inventory only in terms of Colosseum Coins.
In general vendors only buy items of the same type they sell. For example armorers won't buy mana stones and mages won't buy weapons. Also some vendors don't buy anything at all, for example many barkeeps.
History
Players seek out shopkeepers that buy at a high rate and sell at a low rate in order to 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 generally not sell below face value or buy over face value, which reduced the chance of players creating money macros. A similar change was made to disallow selling crafting items (player made) to any vendors. This prevents buying crafting components, making something of higher value and selling to a vendor for profit.
Note there is still one NPC that sells spell components at 100% of face value, but is only available to red PKs (Bael'Zharon (NPC)).