I am thinking of purchasing a copy of Windows 7 via either ebay or GumTree. I am unsure as to how the product key works.

A close friend of mine is warning me against buying it from ebay as he is suggesting that once it has been used, the operating system registers itself on microsoft servers using the serial number of the motherboard of the system where it has been installed. This means once installed on one machine you wont be able to install it on another machine.

Now i am struggling to believe that an operating system can only be installed on one machine. Can someone please explain exactly how this works. I can see a lot of copies being sold on Ebay which are used. I used the 'Ask a question' option and the majority of the users are saying that i should be able to use it.

If someone buys Windows 7 from the shop, installs it on his PC but then decides that he wants to sell it can he not sell it? Will the person buying it not be able to use it? Does the person selling it have to somehow unregister it first? What do i need to look out for if buying it from Ebay?

Thanks