Chest shops have more functionality and look cooler than trade signs
The Trade Signs have some limitations with what can be sold in them so I would highly recommend following this guide to create a chest shop. Its fairly simple and the game essentially guides you though it while creating it.
Hold the item you want to sell in your hand and left click a single chest. A message will come up asking how much you want to sell the items for. If you put in 100 that is equal to 1 diamond, 200 is equal to 2 diamonds etc.
Do not put 'D' or '#x27; in front of the number when you are using the chest shops it does it automatically.
You can change this in a later step if you accidently put the wrong amount.
Once you have entered the amount that you want to sell for it will automatically turn into a chest as pictured below. This is confirmation that the shop has been created.
As owner of the shop you can right click on the sign to make a variety of edits. You can change the price of the items sold, the amount of items sold at once and you can also delete the shop.
This shop is selling 64 spruce planks for $100 or 1 diamond.
If you want to stock the shop you need to right-click on the chest and put the items in directly.