Try Keys4Coins there you can buy in game credits like Steam gift cards, PSN cards, Origin, Xbox gift cards, itunes gift cards and even games itself. It's very convenient to use and so easy but for me, I mostly prefer games that is entertaining, challenging but at the same time earning.
I'm curious about that Keys4Coins it looks legit and useful. That's all I needed, by the way regarding on what you said what are the games you usually play and at the same time earning?