To create a shop all you need to do is place a chest with a sign above in this format:
First line: Blank, your username will automatically appear when you click done
|Second line: Amount you are selling|
|Third line: Price for items you sell : Price for items you buy|
|Fourth line: Item's Name or ID|
Here is an example:
You can buy 64 Diamonds for 10 currency or sell 64 Diamonds for 5 Currency.
And click done! Be sure to fill your chest with the item you're selling!