Where do people take keys?
Having a single game key is relatively easy – in the end, there are plenty of giveaways or competitions on the web. Sometimes owners do not block the ability to unlock one key on a given IP, so people take a few of them to sell – either on Allegro, on Facebook or on G2A.
Some people also follow the principle of “invest to earn”. How? You’ve heard about the Humble Bundle (click!). You can find games there for even $ 1. And then the matter is simple – you sell more expensive! Some people also buy keys from others on G2A or other portals of this type, then sell them more expensive (but still cheaper than the manufacturer).
Some also fear that the keys to buying Steam games through Russian proxy (they are definitively cheaper here). It is illegal and even threatens to compromise your account, but fortunately Steam itself quickly tries to punish users changing proxy. However, buying games from Riosians (who pay less) is not illegal and you can do it without fear. Surprisingly, the Russians are trading pretty well!
There are also those who practice buying keys for money from stolen credit cards to sell them. Some try to trick the keys by referring to editors of known sites or streamers. The problem, however, is not on the G2A side or other platforms, and on the publisher side who does not control who gets the keys. And from experience I will say that even in the case of How2Play requests for the key to review a particular game we were not in any way checking whether we actually create this portal. On several occasions, we have been denied a key because … someone from How2Play already got it, but the review did not show up! In this case, you can only get spoilers for publishers …