Don't forget about the King...
If only my server didn't flag kinguin while going through my virus and malware scan I wouldn't forget about kinguin :P
I have bought dozens of games from them and DABand's aforementioned g2play/g2a(which are sister sites of Kinguin).
Both sites are completely safe for buying keys.
I would suggest however to anyone who buys make sure you buy the Steam Key, not the Steam Gift. The Steam Gift option is the only thing I have ever heard about any type of problems.
I use g2a, and I have used kinguin in the past