Sign shops are for players to create personal shops with the economy system. There are two types of shops, buy and sell.
Creating a Sign Shop
To create your own sign shop, place a sign down, and write down:
[buy] OR [sell]
Here is an example of each:
After that, there should be prompts that appear in chat, follow those. If they don't appear, or are too confusing, you can keep reading. Place the amount of that one item you are going to sell with this sign shop in the chest. So, if you are going to sell one at a time for the price on the sign, have one of that item in the chest during setup. If you want to sell 2 at a time for the price, have 2, etc... (Same idea with sell shops).
Now its time to link the chest and the sign. Hold a piece of redstone dust in your hand, punch the chest then the sign. After that you should get confirmation in chat that they have linked, and the type of sign shop on the sign should turn blue.
Here is an example of a linked, working buy sign shop:
Once the sign and chest are successfully linked, you can stock up your shop by adding as many of that one item in the chest as you like.
You can try purchasing (or selling) from your shop to make sure everything is right. Your job from here is to just stock your shop as you please, and enjoy your earnings!
Other Info & Things to Keep in Mind
The chest with the items inside should be in a protected location, like a town claim.
The sign can be wherever you like and will be 100% protected from griefing.
You can break your sign shop with a golden axe.
To buy from a sign shop, simply punch or right-click a sign.