To start, place a chest on the ground.
Next, place a sign 1 block away from the chest. For example in this screenshot it has been placed above the chest.
On the sign above the chest, write a format of the following:
You know if you have created a shop successfully if you get a message along the lines of "Shop successfully created!" in chat.
If you are having issues what to call an item on the sign in your shop, use the /iteminfo command to figure out what the name of the item is and also what to put on the shop sign, as seen below.
The first line will be your username.
The second line is the amount of items to buy or sell per transaction. For example in Fig 1: Buying 1 Diamond for $10 and selling 1 Diamond for $5
The third line is a combination of buy and sell prices. You need to have a price indicator (like B - price that will apply to items bought by users and S - the sell price) near the price, the prices are separated by a colon :
The forth line will hold the id/name of the item. Note: If you get stuck with this you can write a ? on the sign and the item will be auto filled with the one from the chest or you can click on the sign with the item afterwards.