G2A and Kinguin are both legit - never had any major issues. Only time I had a minor issue was when I ordered ~50 copies of Rocket League....one email and they sent over replacements for the ~10 or so bad codes
Never had a problem with Kinguin.
I had one key get banned (R6 Siege Y3 season pass) and they replaced it within 24 hrs, always worth paying that extra for the shield.