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!