Hello, I have a question regarding Windows 8.1 product keys. I was looking to buy a Windows 8.1 product key card off Ebay. My issue is after I used the product key on my newly built PC, could the seller of the code use it on one of his PC's and therefore, result in the Windows 8.1 software on my PC to not work? Or would it just not allow the seller of the code to re-enter it after it has been used by me? Any input would be appreciated.