OEM versions of Windows can be purchased from retail outlets so they're not limited to pre-installed systems. The difference between an OEM key and a retail key is the OEM key is tied to a given system and cannot be transferred. Thus if I purchase an OEM license from a retail outlet I am able to install that license on one and only one system. Once installed theoretically it cannot be transferred to a new system (even if it is removed from the previous system). A retail key does not have such a restriction. It can be transferred as many times as you want as long as it is removed from the previous system. The problem is that, move it enough times, it won't automatically activate and you'll have to call Microsoft to activate the license.
License activations are tied to specific hardware profiles and not any specific hardware ID as shipped from Apple. The hardware profile (which I refer to the hardware ID going forward) is created from various hardware components installed in the system at the time the key is created. The specifics of how this hardware id is generated have, understandably, been a closely guarded secret.
When you attempt to activate a license the hardware ID is, along with the license key, sent to Microsoft's activation servers. The activation servers look up the license key to determine if the license is already associated with a hardware ID. If not the hardware ID and license key are recorded as associated with one another and, assuming the license key is valid, the system is activated.
If the license is already associated with a hardware ID that hardware ID is compared with the one computed for the activation request and, if they match, the system is activated. This means you can continually reactivate the same license on the same hardware over and over and over and over again. Thus if you constantly had to rebuild the system you do not need to do anything special.
The way the hardware ID is computed is flexible in that it will tolerate changes to the hardware profile and still activate. Thus if you add more memory, assuming memory capacity is used to computer the hardware ID, you can still activate the license (or it remains activated) even though the hardware profile has changed. It is significant changes which will cause the activation to fail (or an existing activation to become inactivated). The extent of these changes is unknown as, again, it is a closely guarded secret.
My personal experience with activating Windows, at least with retail licenses, is there is likely a time based aspect to the activation. I say this for two reasons:
- I have had no issues automatically activating a Windows server license that I have moved to new hardware over time. Never have I had to call Microsoft to activate the license. This is a wholesale move...everything about the hardware changed. My thought is the algorithm becomes more relaxed as time goes on.
- I've upgraded several systems over a period of time and none of the upgrades have triggered a reactivation of Windows. A memory upgrade here, a USB card there, video card then, new hard drive tomorrow. Perhaps if I did all of these at once I would be required to reactivate. But over a period of time I've never had an issue.
Probably more than you wanted to know and, as no one knows the details, perhaps some inaccuracies. But should be helpful.
Back to your original question: One cannot know if a license has been previously activated until they try to activate it. If it has you have no recourse if the seller does not want to resolve the issue. Basically you're taking a chance. But the selling of OEM licenses is not prohibited (as long as they've never been used).
