My understanding is that not even that is safe. Amazon mixes stock for the same sku product from all their suppliers, so even if only one of them is selling fakes, you can get one.
Not even "Ship from and sold by Amazon.com" is safe for the same reason I believe.
To clarify: This is based on experiences I've had en AmazonES buying things directly from Amazon (Gillete razors from the top of my head) and them being obviously fake. Also from previous discussions in HN