There's been a fair bit of reporting recently on how Amazon's system is basicaly set up in such a way that it's super easy for scam-artist 3rd party-sellers to sell items there with virtually no consequences. eBay actually has better seller controls on its site than Amazon does, because, I suspect, as Fox said, Amazon really doesn't actually care that much. They would lose far more money tightening the screws in their system to reduce the chances that bad actors are selling pirated (or bootleg/fake) merchandise than they will with the occasional "oops!" scandal where someone discovers a problem and calls shenanigans on it. I like Amazon as a store and a content-provider but we all know about their less-than-aboveboard practices when it comes to how workers are treated at their firms and how they squeeze out competitors in probably-legal-but-not-necessarily-moral ways. In this way, they're really no different from Apple, Google or any of the other mega-corporations.
The article I read happened to be about fake shoes or purses, but frankly it's even easier to sell pirated software. If, as the Surviving Mars dev suggested, these are just GOG DRM-free builds, hell, any one of us could buy a game on sale at GOG, post it to a server, and "sell" links to it. That's probably an oversimplification of what's actually going on here, but I'm not well-versed in the kinds of techniques that might actually be involved. The point is, it's not hard.
What can you do? Probably just remember caveat emptor, especially when dealing with "deals" from 3rd-party sellers. There's a pretty good chance all of these "insane" deals are "insane" for a reason. Yes, you can report a seller, but that person can just create a new account and continue doing what he was already doing.