Do those all accept third-party keys though? From what I read, some dont. And then theres still the chance of buying bad keys from a third party...
No
https://support.steampowered.com/kb_article.php?ref=7480-wusf-3601
Any of these games can be activated on Steam. When you enter the key, it appears in your Steam library. You can then download the game, update the game, mod the game, whatever, at this point there is zero difference between buying a copy through Steam, or via the key.
When you buy a physical copy of a game that must be activated through Steam, as soon as you walk out of the shop, the disc that comes with it is useless. You activate the key through Steam, Install some of the content from the disc and then the rest will download, along with the day one patches and DLC that are becoming more and more common... Again, at this point there is zero different between buying a physical copy, or through Steam, or via a key.
Aside from that there are other uses. For instance, I have Warcraft 3 + TFT, but I cannot find my booklet that came with WC3. Since you need to activate WC3 to play TFT online, my online TFT is pretty much useless. If I so chose, I could buy a key for WC3 and use it to play. Don't have to go to the shop to get a copy, don't have to wait for it to be shipped, I already have the physical media, all I needed was another key.
There are indeed some dodgy sites out there, but you get dodgy ebay sellers, dodgy shops on the street, dodgy etailers... If there is a way to make money in a dubious way, someone will find it. Just because there is "a guy" selling stolen TV's in a back alley does not make all electronics retailers sketchy as well. The same is true here.
All that has been displayed by G2A is a distribution hitch. Something similar happened to Sniper Elite 3 where a shipment was stolen and the keys sold on. These were caught and banned, HOWEVER, unlike Ubisoft, Rebellion and 505 actually helped out the customers and retailers, not just remove the game with no explanation.
The people at fault here are not G2A, who from what Punk has said have been awesome throughout this, it is partly down to those that dubiously sold the keys, but mostly down to retailers and publishers. If they released the games at the same prices throughout the world, rather than cheaper elsewhere, this sort of practice would not happen, and everyone would benefit. When you pay you 70 bucks for a game, it is being sold in India, Russia and throughout Africa for 20. Explain how buying from a "legit" source is fair when you are getting charged 3+x more for an identical product, and how obtaining it for a price closer to it's actual value is somehow bad?