After you creating a new listing for a product, you can specify it as FBA inventory by changing the Fulfilled by value from Merchant to Amazon.
Change a listing to Fulfilled by Amazon
To change a listing to Fulfilled by Amazon:
When you close or delete an FBA listing, any inventory you have in Amazon FBA fulfillment centers will not be returned or disposed of until you submit a removal order.
Close a listing
When you close a listing, the status changes from Active to Inactive, and makes your product unavailable for sale on Amazon. Your inventory of that product will remain in Amazon FBA fulfillment centers. You can relist your products, submit removal orders, or fill orders through other channels with Multi-channel Fulfillment.
Delete a listing
When you delete a listing, any offers that are fulfilled by Amazon remain in your inventory list, but are not available for purchase on Amazon or for filling Multi-channel Fulfillment orders. Inventory in Amazon fulfillment centers will remain there until you submit a removal order.