Can a user change feedback on eBay six weeks after it was originally given?
I have a feeling that a lot of these vendors get positive feedback because the codes they're selling initially work. It's not until many weeks later that Apple may invalidate the code.
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At one time, the big deal was that someone "cracked" the code that's used to generate AppleCare registration codes, generated a ton of these codes on their own (i.e. they didn't purchase legit codes from Apple for resale -- they simply made up random codes on their own), and then sold them.
So yes, if Apple has any reason to believe the registration code you used wasn't valid, and you can't provide a valid Proof of Purchase from an authorized reseller, then they can invalidate the AppleCare.
There are so many grammar and spelling errors on the webpage you mention above. "WARRENTIES" ... really?!