People pointed out that G2A is just an ebay catering for digital goods. I'm not intimately familiar with the site but I understand it in concept and it doesn't offend me. I think the assumption you're making is that "suspiciously" cheap means it is cheap for "nefarious" reasons. But apparently a lot of it is purchasing keys while they are cheap and selling them while they are higher. There seem to be many reasonable explanations as to why a code might be sold cheaper on g2a, but people assume the worst.
I know people in general love to focus only on the negatives of something. For example, we'll have someone claim they got a code revoked because of suspicious reasons or something. But we don't really hear from the people who use the site and never have a problem.
I've never used the site, but to think someone would defend them only because they are sponsored, or as someone suggests there is a defense force for everything, is a bit insulting to people who legitimately think the site has value for them.
I personally choose to reserve judgment until I know all the facts, at least to a reasonable degree.
The whole issue is that the entire market is "Don't ask, don't tell". Yes, most of the codes come from "official resellers". But no-one asks how they got the codes.
I have countless examples of similar sites buying keys from a "official reseller", and being contacted by the actual publisher, who state it's impossible since they don't sell keys to that reseller.
Practical cases of russian code gifting, opening physical retail copies etc etc. I am not gonna tell you what to believe, and G2A certainly is not the only one with these practices, but believe me when I say that right now this whole market is still a cowboy affair, and the one with the biggest mouth and least moral conscience wins.
The market is trying to shift, but physical retail contracts prevent that from happening fast. It will take some time before we reach a clean digital key market. Some companies are trying to clean it up, and G2A certainly isn't one of them.