Guess I'm lucky, never had any authenticity (or otherwise attributable to Amazon instead of the manufacturer) problem with Amazon parts, including those I bought from Amazon Warehouse as scratch 'n dent. I only buy from sold by Amazon, not a 3rd party. Then again it is not where I buy most auto parts.
I've bought brake rotors, pads, exhaust, upper intake manifold, bushings, spark plug wires, brake line, ABS sensors... I'll stop, there's no way I can remember everything, but none of it was OEM brand. I only pay that middleman markup when necessary, meaning for most wear-parts there is a quality major brand alternative that costs less than 70% of the OEM part, and sometimes it's even the same manufacturer who made it for the OEM.