Bukkit

Exchange signs let you put predetermined buy/sell requests on a sign which are issued upon punching the sign. The price on the sign automatically updates to the going rates of the item & amount on the sign.
Leftclicking the sign will sell the amount of items from existing buy orders.
Rightclicking the sign will buy the amount of items from existing sell orders.

Once the player has punched the sign, EM will ask them to confirm the transaction in chat using the /em confirm command. They can reissue the confirm command as many times as they desire or until the price they are buying/selling for runs out.

To make an exchange sign, put "[Exchange]" on the first line, the amount on the second, leave the 3rd line blank and the 4th line is the item name/ID.
The signs are purposely similar to ChestShop's signs for familiarity sake.