What is the website where is safe to buy steam keys for game ?

Stick to Green Man Gaming or GamersGate

G2A.com is okay as long as you buy directly from them or use the G2A Shield when purchasing from their user marketplace (guarantees a refund if it's a bad key). There's also bundlestars.com for bundles or cheap deals that change up regularly.

GameFly's also safe. If you can find games which you can actually buy if you live outside the UK, of course. Fewer and fewer every time. And most games that get awesome price reductions automatically become unavailable for people outside of the UK because the site's owners are a bunch of cunts. And they become available again once they get back to their usual price. Awesome, huh? Same thing with nuuvem.com. But the beauty of nuuvem is that it has very low prices characteristic to Brazil, even without sales. When it puts a game on sale, it is truly an awesome deal that should not be missed(if you can buy it).

I use allkeyshop.com which lets you compare game prices and then buy the cheapest (It's a referral service). I've bought 10 games from sellers and not had a problem yet, often 50% cheaper than Steam. All you need to be sure is that what you are buying isn't region locked, but it always says in the description.

The ones listed on http://www.steamgamesales.com/ and Amazon =)

GreenManGaming, GamersGate, Amazon, Newegg, Desura, Humble Store, IndieGala Store come to mind. There's an insane list over at http://isthereanydeal.com. In general, you can trust an online portal for a store that has physical locations as well (e.g. frys, bestbuy, etc) though they don't really have the best prices.

Kinguin.

http://www.gamekeysnow.com/

Avoid the grey market (kinguin, sites like ebay but for keys etc). No one really knows the origin of the keys, they could be stolen, stolen/taken from retail boxes etc. If you do use the grey market and get ripped off or the game removed from your library its would be your own fault. Sites like G2A have been known to sell illegitimate keys https://twitter.com/devolverdigital/status/466577590606520320. Why take the risk?

Stick to authorised steam key resellers, GetGamesGo, GamersGate, GreenManGaming, Steam, Amazon, Humble Bundle store. There are probably some more but I only use those.

And TF2outpost ?