You can turn any chest into a shop that lets you sell items to other players. Simply left click your chest while holding the item you want to sell, and then type in chat how much you want to sell the item for. You can also create a warp that other players can use to visit your shop.
/shopwarp set- Sets your warp to your location/shopwarp name <name>- Sets the shop name/shopwarp icon- Sets the shop icon to your tool/shopwarp remove- Removes your shop warp
All shops are accessible via the portal at spawn, simply click the button and select the ‘Player Shops’ category