The vending machine is an uncraftable item which can be obtained through other players or you can buy 3 for 1500 cubits on the cubit store.
It is used by placing a glass block with items in it above the vending machine and you set a price for how much you want to sell your items for. This is useful if you do not want to sell items through a cash register or so you can sell items when you are not at your realm/offline.