If the key passes genuine validation, then it is a legitimate product key. But the way in which the merchant or vendor obtained the key may or may not be legitimate. This is a concern you should have when trying to purchase product keys online. Retailers like Amazon.com will not sell you volume license keys, but will sell you "Anytime Upgrade" packages. You should look into this with extreme caution. It may very well be that the key is legit, but that the means of providing the key violates Microsoft's license agreement. If this is the case, the key itself could become blocked in the future if it is found to be distributed under illegal means. This can be from theft, from distributing MSDN or TechNet keys, or obtaining leaked keys. White collar crime is believed to occur more than regular street crime. There is no way you are going to find a legitimate copy of Windows 7 for $15.00 at this time. I would advise against it, as there is 1) no way to determine if the key will even work and 2) no way to determine if you are buying a stolen good.