Multi-Channel Fulfillment. Why do this? You'll never have to slow down your workflow due to your own inventory being out of stock.
Go to the Integrations tab and select Channel Integrations from the drop-down menu.
Under the Inventory sources, check the boxes for both FBA and Fulfill orders.
Make sure to click on the Save Integration button at the top of this page and you are all set. Now, when you receive an order for an item that you are out of stock, Listing Mirror will automatically use Amazon's warehouse stock to fulfill the order.
In addition, here are some related articles you might find useful.