I only order items "Sold by Amazon" and have never received a counterfeit item (AFAICT). I always assume those reviews are from 3rd party sellers -- is that an incorrect assumption?
Amazon commingles everything, even "sold by amazon" products, unless they've changed that recently. Everything goes into the warehouse by SKU and has no further tracking.
I've heard many anecdotes to the contrary. At this point I don't know what's true about it or not so I just choose not to trust Amazon until I can be convinced otherwise.