The best way to avoid this is by monitoring your completed and pending orders and keeping your listings updated to reflect real-time inventory quantities. However, there are instances when inventory management issues may lead to multiple sales of a single item.
For example, when a buyer initiates a purchase for a single-quantity listing but payment is not immediately successful, the listing may still appear on Manage Inventory with a quantity of "0" while payment is pending. If the listing is then edited, this will add a quantity of "1" and bring the total quantity available for purchase to two. At that point, two buyers could submit purchases for the listing in two different orders.
If you have only one item available, you will need to tell the second buyer that the item is unavailable, and then cancel the order you are unable to fulfill. In addition to updating the order status in the buyer's Amazon account, we automatically send an e-mail message to notify the buyer when you cancel an order. Keep in mind that buyers can leave feedback for orders that have been canceled by sellers.