Earlier quoted context omitted.
Amazon does SKU aggregation in their warehouses for all the sellers that use Amazon fulfillment. Every delivery of that SKU, by whomever, gets thrown into the same pile from witch they pick whenever someone orders said SKU through whoever is selling that stuff on Amazon. This means that the product you receive from seller X was probably not delivered to the warehouse by seller X, but would be part of a batch delivere…
It wasn't always that way; it was a conscious change they made to save money, which they decided was more important than being able to do better quality control when counterfeit products are discovered from a certain seller. I just think it's important to point out it's how they chose to do it, presumably knowing the potential downsides, rather than some impossibility of warehouse operations.
Sure they have a good return policy but who wants to deal with it or assume they’ll always honor it.