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eBay is known to have cheap AppleCare plans. If the seller has a high feedback rate and has sold AppleCare plans before, then I would assume it's safe.
 
Bought for my 2.16 MBPro a while ago from a power seller with good ratings, no problems at all :)
 
yes

Have done so myself with no issues at all. Make sure the listing says its an unopened box.

All you do is take the registration serial number for the item, enter it on apple's support website, type in the serial number of your machine, and you're good to go.
 
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