I use G2A alot but trust Kinguin more
Some warning about G2A - I think some people have been pissed off after getting used keys so have created many different accounts
trying to resell those bad keys (Hot Potato) - either that or they are following the dodgy sellers around trying to make them look bad by hosting a lot of sales with theirs from users titled "There is no way", "Me Again", "Lose Lose I Win", etc.
Don't spend big $ on G2A and even if gambling small, considering the war taking place on that site, I really wouldn't trust any seller with less than 100 sales and 100% rating under their belts