You must contact your place of purchase. Please review Frequently Asked Questions about Warranty Information for Intel®...
Who do I contact to see if the warranty covers my problem?
OEM processor family Contact your system manufacturer or place of purchase. Check the warranty documentation that came with your system for specific warranty terms and conditions. Intel® Boxed Processor family Contact the system manufacturer, original place of purchase, or Intel Customer Support.
Personally speaking, when you purchase a used processor online, especially a tray/oem processor, you should make certain there is a refund policy in effect from the seller, and your expectation should be set as low as possible.
"The seller disappeared" is part of that "low" expectation that I was talking about. As always, online, or with any auction site, the rule is "buyer beware".
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The information posted by Doc is excellent, also, please bear in mind that the warranty on an Intel® processor only applies to the original owner. Second hand units are not covered under warranty.