I always press the "Seller: Newegg" button. This way I never get counterfeit stuff. On the other hand, specifying the seller to be Amazon on Amazon.com sometimes yields third party products in disguise.
Amazon suffers from that as well. I'm loath to buy anything from Amazon, Newegg, or any other retailer when it comes from a third party seller. As far as I know, that's always indicated on the product page. They don't hide it, but they don't aggressively inform you either.
Since Amazon commingles inventory, it doesn’t matter if it says shipped and sold by Amazon since the item you get could still be supplied by a third party.
I was just comparison shopping for an HDMI adapter across Amazon, NewEgg and Bestbuy. Out of the 3 I found NewEgg still to be the best even with the 3rd party sellers mixed in. Amazon the quality looked super sketchy for some of this stuff and for Bestbuy the website had loading problems, an unintuitive UI for store selection (and put me into a store in CA for some reason even though I'm nowhere near there and had no VPN).