There is a reason Steam censors 'g2a' in their discussion forums, when mentions of legitimate key re-sellers (like the Humble store) and competitors are fine. At best they sell grey market keys bought in lower priced regions. They have also sold keys originally purchased with stolen credit cards, and a few months ago a bunch of people had games removed from their Steam libraries for that reason.
Also, 'spammed'.
I've mentionned this somewhere else too, but I got a reply from G2A:
"Greetings,
We are happy to inform you that the complaint process is completed and your claim was accepted. The order will be refunded, you will receive a confirmation email soon. We would like to apologize sincerely for the inconvenience you have experienced.
Kind regards,
Michael D. "
So it would seem they take up their responsibility, and everything will be allright.
It also strengthens me in my believe G2A definitely is a legit site, which also deals in legit trading (well, selling) of legal keys.
As I've pointed out, you *do* have the right to resell things you bought, even when digital downloads, in spite of any terms of service. So it's legal, and they seem to handle things and act as a responsible company too. So I really have no major complaints about it. I think they were as much a victim of the card-fraud as I was.
