I bought a game from them last week with no issues. After reading reviews, it seems to be recommended to buy directly from g2a and not from their "marketplace". They allow other people to sell games on their site as well..
I've bought many games from G2A and their marketplace. I love that site. Had a bad marketplace key once but used their guarantee to get a refund without issue.
One of the best thing is the ~$1 Bitdefender 6 month licenses and cheap AV.