I've bought tools from Amazon fairly recently. They're things that you could totally counterfeit if you were interested in doing so, but at the same time they're things that I can immediately identify as being either legitimate or indistinguishable from such. Like, I purchased a set of iGaging double squares on Amazon and tested them against my surface plate and my Starrett square. They pass. Maybe somebody's counterfeiting the thing and selling it at the same price to me, but it's also not a worse product, so I couldn't tell.
I don't doubt that there's a lot of counterfeiting going on, because it's the internet, but it probably depends on what you buy as much as anything else. I'd be much more worried about substandard quality from buying things from the bajillion sellers with what sound like generated names (because they all look the same but did this one swap out component X for one that's a couple cents cheaper?) because there's no good way to control for it--but in those circumstances Amazon's return policy is also pretty great.