Do you have any link or evidence of any kind to prove that G2A sells keys illegally or takes part in any sort of dubious business practice?
TotalBiscuit did a whole video explaining how these gray-market CD key resellers have no legitimate inquiry into where they source their keys from; how very little of the profit they make on games goes back to the original developers (if any at all); and how they exploit regional pricing systems to sell games outside of their intended market, defeating the entire purpose of regional pricing/price locks in the first place. (Yup, buying keys "in bulk" and selling them against the publisher's specifications sounds totally legit to me.

)
As TB tweeted:
G2A and their ilk are unauthorized key resellers who, at best, source from cheaper regional markets. Use them at your own risk.
At worst,
G2A sellers will spam game developers with emails claiming that they are a major Youtube review site and want to have a free copy of the game, only to abuse the dev's good will and flip the game on G2A for profit.
As one dev states,
I expect I would have lived in ignorance for a long time if it hadn't been for one thread that appeared on the Steam forums. Some folks reported that Worlds of Magic was available for purchase for about 15$ at [G2A]. I was sure that something wasn't right as the price has been set up everywhere to be exactly the same... So, as it turned out, roughly 70% of the keys we had given out were taken under false pretenses, or to use a more direct term, stolen.
There was a period of time when, literally, many of these key resellers were
profiting off of charity and bulk-purchasing Steam games from sites like Humble Bundle to resell them at a higher price. Sites like G2A are the reason why there is now $1 minimum requirement for Steam keys, or BTA prices, or activation links instead of Steam keys, or one link for all games so you can't reuse keys/links for games you already own. They are, in a very serious way, the reason why we "can't have nice things" anymore.
"I do know some indies who also had problems with their games being sold without their agreement on that site,” Keys says. Games are sold direct from G2A and by users of the site. For example, Thomas was Alone is selling for as low as $0.97 from a member of G2A’s community.
It was already mentioned above, but yeah,
Rebellion cracking down on gray-market copies of Sniper Elite III was a very real thing, and thousands of games were removed from people's libraries.
We are receiving a lot of reports from you guys telling us that Steam is currently revoking Sniper Elite III keys bought from sites like CDKeys and Kinguin. Valve has yet to offer an official explanation or statement but this could be a sign that the company is going to fight back against cheap CD key selling sites... Some retailers are offering a discount on your next order to make up for the trouble but we don’t know if this is Valve’s first step to ridding itself of unofficial sellers.
It’s a little early to say whether or not Valve is moving to ban games bought from sites like G2A, CDKeys or Kinguin but it’s possible. Hopefully Valve will offer a statement soon.
If you were one of the people naive enough to purchase a copy of the game from one of these sites, the message Steam gives you upon revocation looks something like this:
Your recent product code activation has been revoked due to a problem processing payment for this item. The games associated with this product code can no longer be played as a result. This could be due to a problem with the payment methods you used to acquire this item from a third-party seller; or a problem with how the third-party seller acquired the product code.
Devolver is another games publisher who is making it perfectly clear that they are cracking down on gray-market sites.
http://www.pcgamesn.com/devolver-digital-games-being-sold-illegitimately-online
Also as already mentioned -
Ubisoft has begun pulling games from people's Uplay's accounts that were purchased from gray-market sites as well.
Your keys were removed because they were found to be illegal. This means they were either generated, bought with stolen credentials, or the key was sold to you by a third party in which they filed a chargeback.
Either way these keys from these 'discount' retailers are NOT supported by the publisher/developer and are often times ILLEGAL. Most of these prices are sold at a total loss anyway, and it only gets money for the seller as they keep your money but find excuses to not refund you.
While these sites may legitimately "purchase games in bulk" from a "supplier", they ARE NOT properly authorized to resell these keys.
THEY DO NOT HAVE A PROPER DISTRIBUTION AGREEMENT WITH THE PUBLISHER TO SELL THE CODES ON THE INTERNET. For most of these sites, this means buying cheap physical copies of games in bulk from cheap countries, and scanning the codes to sell online. (My favorites are sites like CJS CD Keys where they make it so damn obvious, they literally just email you a camera photo of the key on the back of a gamebox someone took in a warehouse somewhere.)
Also, while writing this post, I realized that writing out the full name of G2A's website is automatically censored by Anandtech's forum filter, presumably because the mods are already aware of their illegitimacy, or because of how spammy people are with their affiliate program. Either way, they suck. Their illegitimacy is one of the reasons why they are met with hesitation by communities such as CheapAssGamer and
outright blacklisted by places like /r/gamedeals.
tl;dr: If you want to save money by purchasing game codes from third-party sites, you should purchase them from authorized retailers like GMG, GetGames, GamersGate, etc, etc, and stop contributing to shady gray-market sites that sell games against the publishers' demands while ultimately making game devs tighten up on DRM even more and ending up making things worse for the rest of us.