We've recently deployed our Shop GUIs, and they can be quite difficult to set up if you don't know what to do. If you have any problems along the way, feel free to comment on this discussion.
Step 1 The Stock Chest
Dig a 1x1 hole into the ground and place a chest inside of the hole.
Step 2 The Stock
Look at the item you're planning to sell by hovering over it in your inventory.
Step 3: 1 Getting Your Item ID
If your item DOES HAVE a little "NBT:" symbol at the bottom, DO NOT follow this step, skip ahead to Step 3: 2.
Your Item ID is going to be where on your item it says "minecraft:". In this picture, the Item ID is "salmon".
Step 3:2 Getting Your Item ID
If your item DOES NOT HAVE a little "NBT:" symbol at the bottom, DO NOT follow this step, skip ahead to Step 4.
In this case, your Item ID is not defined by anything in default Minecraft. So, we're going to have to make a custom one. Go ahead and hold the item you're planning to sell in your hand, and type the command "/create_item". You'll get a message like the following in the picture, and your Item ID will be whatever it returns. In this example, the Item ID is "item#1".
Step 4 Setting Up Shop
Go ahead and place a sign of any top on top of that initial Stock Chest. On the very first line, type either "[Buy Item]" or "[Sell Item]" depending on if you're selling or buying. Keep in mind that this is the decision for the player, so "\[Buy Item]" means that the player is buying the item, not you, you're selling it to them.
Step 5 What Are You Selling?
On the second line of our sign, go ahead and type the Item ID that we figured out from earlier.
Step 6 How Much?
On the third line of our sign, go ahead and type how much each of your item will cost. Keep in mind that this is how much a SINGLE one of your item costs.
Step 7 Who's Selling?
Lastly, on the fourth line of our sign, go ahead and type your Minecraft Username.
There it is! Go ahead and press done. If you've done anything wrong, it will tell you in chat. The finished product should look just like this.