If an order only has inventory (i.e. Items to Sell), it’s treated as a Sale Order. When you hit the Rent Out button, the order is marked completed and the inventory is sold as per the quantities and pricing details in the order. Such an order doesn’t go to a Rented Out state since it doesn’t have any rental items (i.e. items that are to be returned).
Learn more about creating a sales order in EZRentOut.