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Personally I bought my AppleCare at a proper Apple Authorized Distributor, because I have heard bad things about codes from Ebay for Applecare. Sure, I payed more, but I can be guaranteed that it is genuine.
If the seller looks legit and the pricing isn't too excessively cheap, it could be legit, but that is a risk you take by yourself.
 
While I am not a fan of extended warranties, I would not buy Apple care from the Bay.
 
I wouldn't take the risk, just buy a legitimate version. You can wait until the end of the first year and still be eligible to purchase AppleCare if you want to save up for a bit. You're covered under the 1-year factory warranty until then.
 
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