How can 3rd party vendors sell Steam keys for mind bogglingly cheap beyond what you can find on Steam sales? Noobs constantly ask on Reddit whether or not such businesses are legit. Humble, Fantatical, Greenman Gaming, Indiegala are definitely legit with direct partnerships with publishers. Game devs and publishers however detest gray market sites like G2A which I personally do not use. A game dev said they would rather players pirate their game than give money to G2A.
I use gg.deals/isthereanydeal.com to track historical prices and past bundle history to figure out what to buy. I will cover 3 sites where I get my cheap games, mostly bundles when I actually have to buy stuff.