It's sad that there's no easy way to know. Humble gets their keys from the developers. GMG, so far as I know, does, too. G2A doesn't get them from the devs.

It's sad that there's no easy way to know. Humble gets their keys from the developers. GMG, so far as I know, does, too. G2A doesn't get them from the devs. They're a lot cheaper because the dev isn't setting the price. Whoever is obtaining the keys (illegally or otherwise) is setting the price there. I look at it this way: If the price is too good to be true and you don't already know the site, it's probably not legit.