A point to raise?
What was in the computer, before your installation? As said by Joshefur, it is possible that your computer already had registered your right to a digital (free) update to Windows 10.
In this scenario, it is important, in the very initial stages of the installation, when you are asked to put in a key, that you respond to the (I do not have a key). At the end of the installation, in that case, you should fins the OS has been successfully activated.
As an aside. The market for already used keys is rife. Personally, I would NEVER buy a key from a third party so