People are getting confused, so lets go into detail
1: Place a chest

2: Put an item in the chest. [first put in the amount you want to sell at a time, then, put in the rest of your product.]

3: Make the sign.
Top line will be [sell] if you want people to sell YOU items. [buy] if you want players to buy items from you.
Second line will be the name of your product.
Third line is the quantity of said item
bottom line will be Price: (insert price here}

https://gyazo.com/c1b53a774fbd5a36fa3f543b15dcf457 Example of a working sign

4. Holding redstone, Punch the chest FIRST, Than punch your sign. The [buy] or [sell] will now be highlighted blue! You did it
5. Put any FULL block over your chest so no one can take anything from you.

Extra things to know: If the buy or sell is red, that means you're out of product or money
Shops can only be broken by holding bedrock, so you have to contact a senior mod + to break a shop.

A video guide of this will be uploaded shortly!