Backstory
Me and some buddies were playing on our server when we decided that we should have an economy with slimeballs as the currency. We didn't want the same amount of slimeballs being circulated around, so we wanted to be able to use a command to sell our items for slimeballs. However, none of the plugins I found on my search for one were cutting it. Something that could've been what I was looking for was BarterItems (Not sure of the name.) However, the GUI, while fancy, was poorly documented. All you were told to do was make a sign with the first line as [barter] and let the plugin do the rest. So, being a plugin developer, I decided I had to make one myself.

Usage
When you first open your server with ItemTrades on, you'll be told there was no configuration found, and that an empty one would be made. You don't have to edit the config file in a text editor, and as a matter of fact, I don't recommend it and wont explain how to. By default, nothing can be sold to the server, so you'll have to set prices up on your own. But first, you'll need to use this command:
/it setitem <item ID>

This is the first command you can ever use, and as a matter of fact, none of the other commands will work until somebody uses this one. All it does is sets the "money" item to whatever item ID you specify. Once you've specified that, that's the item that will be used as currency. Next, you're going to want to let the server buy things, right? So what you'll have to do is set the prices for the items you want players to be able to sell. What you'll have to do is hold it in your hand, and use this command:
/it setworth <value>

This will set the amount of the money item your players will get for each piece of the item your players are selling. So if I wanted them to be able to sell TNT for 5 slimeballs each, I'd have to do this:
/it setitem 341 to set the money item to slimeballs.
/it setworth 5 while holding TNT to set the value of TNT to 5 slimeballs each.

Now, for players to sell their items all they have to do is hold the item in their hand, and say /it sell. If a value for the item has not yet been set, it will give them an error saying the server isn't buying it at the moment.

As of version 1.8, users can now use /it worth to see the worth of an item.