You can create "barter shops" that trade one item for another when a player uses the shop sign. Barter shops make it easier to trade and it also prevents people from stealing your items.
You can crouch-right click (default key: SHIFT) a shop sign to trade items. When you first create the shop sign, the [Shop] text will be black, but once someone uses it, the text will turn green or red indicating whether the shop is in stock.