How to set one up
To set a chest shop up once you have the tools is simple, you just type on the sign the correct format depending on which type of shop you are interested in creating. Whether that is a buy shop, a sell shop, a combo shop which allows for both the purchase and sale of an item, or a barter shop:
BUY SHOP SELL COMBO BARTER
[shop] [shop] [shop] [shop]
Quantity Quantity Quantity Quantity item 1
Price Price Buy/Sell Price Quantity item 2
Buy Sell combo barter
In these formats, the quantity is the amount of the thing you want to sell at once. Price is the cost to buy/sell the amount specified in quantity, and the last line specifies whether you are buying said item, selling it, doing both or bartering. After the sign is made properly, it will show in red on the chest, that is when you give it the item you want the shop to be about. You give it the item you want to sell/buy by left clicking the red chest sign with the desired item on hand.
If you did it correctly, the sign will change accordingly to show the item you have selected as being in the sign for the chest shop. From there you can fill the chest with the desired item to sell, or leave it empty for buying from others.