From MothCraft

You can sell things to the server, all worlds and everyone can see it. open the mainmenu item by right clicking with it, then clicking "shop menu" and then "player shops" at the top. it will list all player shops and tell you how to create your own at the bottom. The gifs on this wiki entry will also show a step by step of how to make your own shop and add items to it. You can also get to the player shops by doing /playershops or /market as a command. If you like making a physical shop in the world, you still can! place a sign on a chest with "[PlayerShop]" on the first line! if you place "[PlayerShops]" on the first line (with an s) you will be directed to the list of all player shops.

Make a sign like this with "[PlayerShop]" on the first line and your shop name on the second and it will allow players to access your shop.
Players are able to create their own shop with a simple button.
Next they can add items from their inventory by simply clicking them and entering a price in chat.
Now as soon as they press the "Save" button, their shop will become public and other players can purchase the items. One big GUI lists all available shops - sorted by permissions or other configurable settings.