People, you realize this is a 2 year old thread that got necroed, right?:/
On topic, I just get most of my stuff from Fanatical since it's pretty much straight from the publisher and has sales/cheap bundles all the time! HumbleBundle is reliable as well, although not as many cheap quality stuff as it used to have back in the day... IndieGala is reliable since it was mentioned and is a nice place to find very niche/unknown indies.
As for G2A, the problem isn't keys not being legit, it's that it has been proven to be one of the biggest money laundering websites around where people sell keys purchased using stolen credit cards. Basically they steal credit card info or buy it in dark web, they buy a game from a keystore, then sell it for a bit less on G2A, thus laundering the money without it being traceable. Also things like site security being among many other issues are just a few reasons to avoid that place... Better to pay a tiny bit extra than to help out money laundering honestly, because if it keeps going on it will spread in more stores as well!