>Additionally, Amazon is offering an optional product serialization service for companies that would have them put a unique, Amazon serial code on their products during manufacturing that will then allow Amazon to scan and confirm that a given products is authentic before it ever leaves a warehouse.

If the manufacturer opts in to this serialization, as a customer, co-mingling wouldn't be an issue. If a co-mingled counterfeit item gets picked, it wouldn't leave the warehouse. The Amazon worker would then put the counterfeit item to the side and find a genuine one.

For the manufacturer/Amazon, it would still be difficult to find who shipped the fake item.