They don't commingle inventory. They know exactly which supplier/seller sent in which units. Here is a quote from their seller support article about the topic:

They don't commingle inventory. They know exactly which supplier/seller sent in which units. Here is a quote from their seller support article about the topic: "Used by default for eligible products, manufacturer barcodes use virtual tracking to trace the source of the products throughout the fulfillment process. Identical items from different suppliers are not physically stored together in a fulfillment center. However, Virtual tracking allows Amazon to fulfill orders using identical products from different suppliers. This enables us to process a customer order more efficiently and expedite its delivery from the fulfillment center closest to the customer." So they will use inventory from different sellers/suppliers to fulfill an order, but they can trace counterfeit/incorrect inventory back to the source if there's an issue.