That can be a bad assumption.
Contact the seller IMMEDIATELY for the sticker or the original Product Key.
If the seller is unable to give you originals, assume it is NOT legit.
Activate after each and see what happens after rebooting:
Start/Run
Type "oobe/msoobe /a" ENTER
Follow the prompts
Reboot to Safe Mode:
Reboot tapping F8 each second.
Select Safe Mode at the menu
Start/Run
Type "regsvr32.exe regwizc.dll" ENTER
Type "regsvr32.exe licdll.dll" ENTER