If i buy

Hmm, there are many. I buy many bundles on HumbleBundle and IndieGala (sometimes also Groupees and BundleStars). If you want some AAA games, then you can wait for Steam to have some discounts (Summer and Winter sale mainly). You can also buy on Nuuvem, they have great prices, but some most wanted games (like GTA V), are region locked there. Then there's always GamersGate and GreenManGaming, however, I use those two really rarely, since they changed their currency from dollars to Euro/Pounds. If you are Polish like me, you could also check out Muve, they had a nice spring sale recently, and I've bought several games there.
If you don't mind trading, then you could always just hit SteamGifts.com/trades and buy the games you want for several CS/TF2 keys (or sometimes pay with Paypal).