hey sorry for my late reply. Don't know if you've worked it out already.
Shop Sign:First lign [Item you want to sell/will buy]
Second lign Buy x for .....
Third lign Sell y for .....
If you do it correctly the fourth line will display your username.
(The ...... can be replaced with any number of i = iron ingot. e=emerald, d=diamond)
Note: some items have a long name and don't fit on the first line of the sign: [Diamond Pickaxe] doesn't fit, so you would need to use the item code: 278. Item codes can be found here:
http://www.minecraftinfo.com/IDList.htmYou can sell enchanted items with this system, though you need a chest for every kind of enchanted item.
You can't sell enchanted books trough this system.
Example[Diamonnd Sword]
Buy 1 for 3d
Sell 1 for 2d
(And here automatically will stand) __Laurens__
[278] (278 is the item code for diamond pickaxe)
Buy 1 for 4d
Sell 1 for 3d
If you want two single chests next to each other you can put a trapped chest and normal chest next to each other.
Good luck!
Laurens