http://dev.bukkit.org/bukkit-plugins/chestshop/
This is the website for it. Niah explains it well, though here is the format for it so you can create your own and test it out.
The first line is your username. For me, I'd type Exclaimer999.
Second line is the amount of items you want to sell/buy at once. Say I want to sell 16 fish at once. I'd type 16.
Third line are the prices. This is a bit tricky, and needs to be typed accurately to be registered. You list the buy price, then the sell price. Say I want to sell 16 fish for 50 Radiants, and buy 16 at a time for 25. I'd type B 50:25 S
The B stands for items bought by the customer. S stands for items sold by the customer.
The fourth line is the item. You can use the item name, or its data value. For fish, I'd type Raw Fish.
In all, it looks like this.
Exclaimer999
16
S 50:25 B
Raw Fish
To use another shop by another player, you left click the sign to sell items to the shop, and right click to buy from it.