You can buy and sell items from anywhere in the game.
To buy items, you can use "/shop" or "/buy".
"/shop" will open a Menu where you can view the various players' shops and browse their items.
"/buy" allows you to search for a specific item being sold.
To sell items, you can use "/sell". This will open a menu where you can place the item(s) you wish to sell and specify the price. Other players can then purchase the item you listed.