They buy the game in countries where it's cheaper and they just resell the keys.
So no shipping, so it's cheaper, and the games they buy are legal.
For example, if you buy this game in the US it will cost 49.99$ but if you buy the same game in Germany it will cost 49.99 EUR So they...
http://www.cdkeyhouse.com/steam-keys/312-xcom-enemy-unknown-cdkey-steam.html
Can't wait for this game to come out! Get your preorders now haha, $20 off:D
The site is legit. I bought Bf3, Dark Souls PTD Edition, and Borderlands 2 so I can vouch for them.