Wondering how to make a chestshop? Hopefully this tutorial will help. So the format is like this.
Then with an example it will look like this. You can change the amount from each purchase, quantity, and the buy and sell amount. But if you dont know the id number, you can put a "?" where the item would go, then it will prompt you to right click the sigh with the item you want to sell. Then just fill the chests and there you go, you have your own market.