"With the exception of Product Key Cards distributed with Certificates of Authenticity (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, it’s a good indication that these keys are likely stolen or counterfeit" - Source
https://www.microsoft.com/en-us/howtotell/whattolookfor.aspx - is a great resource offered by Microsoft to protect you from counterfeit software.