The Market Box is a place where you can list items for sale, which are able to be purchased by other players. Once you list an item for sale in the Market Box, you cannot remove it or alter the price. You begin with four slots for items, but will unlock more as you level up. You can also purchase more slots for Leaf Tickets.
You can view the first four items a friend has for sale from the Friend List, but you need to visit their Campsite in order to see the full list, or make any purchases.