Recently there has been some debate on the legality on the buying and selling of Microsoft activation key's. Luckily for us Microsoft are quite clear about this:
With the exception of Product Key Cards (PKC’s) distributed with COA’s, Microsoft does not distribute products keys as standalone products. If you see a listing on an auction site, online classified ad, or other online page advertising product keys for sale, it’s a good indication that the keys are likely stolen or counterfeit. If you were to purchase and use a stolen or counterfeit product key to activate Windows installed on your PC, the key may not work for activation, may already be in use on another PC, or it might be blocked from use later by Microsoft when the key is reported stolen. The best way to get everything you expect up front is to buy Link Removed or Link Removed.
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Users may have bought keys that work but basically they are not legal.
Any threads or posts relating to the selling of keys will either be edited or removed as spam.