I ordered a pair of SD cards from a FBA Amazon seller which turned out to be counterfeit. I used Amazon's contact form to report these, and was told to just initiate a return.
I think I chose "Item not as described" as the reason in a drop-down, and entered "Counterfeit" in a comment box, but someone later changed it to some benign-sounding reason. I got a refund on my credit card, but now have the impression that Amazon just does not care. That experience and stories like this here convinced me to first look elsewhere when I need something.
If you want to ding the seller further, leave a negative feedback response (for the seller) on that order.
Mark the reason as "item not as described" and don't add any additional text that would allow the seller to claim the feedback was "product related / a product review".
Speaking from experience as an FBA seller for the past seven years, those are nearly impossible to dispute with Amazon's seller support staff...even when the buyer is incorrect (ie they contact you via the messaging system and admit to buying the wrong product or misreading the product description). Enough negative feedback ratings and the seller's account can go under review and/or be closed.
If you wanted to be sure it was warranted, take a look at the seller's feedback prior to leaving yours. If there's a hint of buyers with similar issues - fire away with a clear conscience.
If you've opened a dialogue with the seller via the messaging system, you should attach a picture of the item with its packaging. IF the seller is innocent (and consistent in their processes re: labeling), they might be able to open a case with Amazon along the lines of "this was commingled inventory - we always apply our label vertically over the item's barcode and our customer received an item packaged with the barcode applied horizontally...therefore you sent the customer another seller's inventory."
That would be a semi-tenuous case to make, but would have a flying chance of working, particularly if the seller had an inbound shipment for the same SKU that was due to arrive at an Amazon warehouse and had left prior to the customer's issue surfacing.
In general though, if there's a shred of doubt...don't worry about the innocent seller. The seller (hopefully) has enough sales volume that your feedback will be lost in the crowd. If not, well, maybe they aren't a good fit for Amazon and you're doing everyone a favor.
I think I chose "Item not as described" as the reason in a drop-down, and entered "Counterfeit" in a comment box, but someone later changed it to some benign-sounding reason. I got a refund on my credit card, but now have the impression that Amazon just does not care. That experience and stories like this here convinced me to first look elsewhere when I need something.