[quote name='JostVice']Why would steam revoke it? I mean, what are the reasons for steam to do that? stolen keys only?[/quote]
Well, that could be one reason...if it's actually stolen...
or selling under value is another reason too?
Regional pricing can be MUCH different in different territories - especially if you're from the USA and you're buying outside of there in a totally different currency (i.e. pounds).
Also depends on what kind of key you're getting, too - i.e. if it has any sort of region attached to it.
Sometimes they do; sometimes they don't.
Usually, that's enforced by either Valve; and/or 3rd party publishers; and/or game distributors.
EDIT:
[quote name='Mooby']Not like GMG is the only reputable site to get Steamworks games/keys from. Amazon, Gamer's Gate, Gamestop, Gamefly, and Getgamesgo are always reliable third party merchants.[/QUOTE]
And if you live in the USA, they usually sell you keys priced in your own currency of US $, too.