This seller has 100% rating and he has sold other Apple products before. I have asked him more detail about the Applecare and he has replied me with this.
"hi there mate. you have NOTHING to worry about. i have sold an
applecare before, if it helps, check my profile feedback. Once
again to clarify, the box at the moment is NEW and SEALED. i
will have to slice the top and record down the actual enollment
number though, as i will get scammed. you could very easily
post on ebay that the enrolment number had already been
registered, and request a refund, in which as your proof you
supply your mates applecare number, which had been registered
before, and seeing as how the box is sealed i will never know to
prove otherwise. And you get a refund and an applecare free of
charge. However, if i record the number down, and an incident
like this occurs, i will have the registration date and details of
the person (which would be you or a spouce of) and you wont
get a refund and scam me in return ! Believe me this works. its a
security and safety measure for both me and you. No receipt will
be issued with this. Apple care number CAN NOT be faulty. Also,
there is a no refund policy with apple with applecare's anyway. A
receipt will be no good to you. If it is for tax purposes, you can
simply use your paypal invoice. I have done that before many
times. This was initially a gift to me from someone who knew i
had just recently bought a Macbook Pro, but i dont think i will
be needing it at all. Let me know if i have answered your
questions, any more queries please dont hesitate to ask."
However, at the end of the day, if there's any dispute, he will say I have already opened the box and cannot return it. (He doesn't accept returns according to his advertisements). If I insist that I haven't use the number in the box, I guess I won't be able to prove that. So I need to pass this purchase I guess. What a shame...
Don't get me wrong. I think he's a decent guy but there's certain amount of risk here.