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I do not know that seller you indicate but he has 100% positive evaluations and eBay offers buyers paying through PayPal a certain protection.
Better not to need it, of course, I had no terribly good experiences with eBay's so called "customers service", but it is better to have any protection than having none.
I can only say that I bought my AppleCare from someone I did not know at all in Germany offering a sealed pack indicated in the picture "Mac Pro/ Power Mac", therefore issued several years ago when there still were Power Macs.
I paid about the equivalent price this seller you mention asks, although in Euros.
Apple had first confirmed me by phone that 1) there is no new AppleCare specifically made for the nMP and 2) that I could buy it from a seller in another country and register it in my own. (I bought my nMP here where I live, in Switzerland).
There was no problem. I paid, received the parcel with the entirely sealed box, opened it, found inside a code and used it to register the AppleCare of my nMP on line.
Everything went -in my case- perfectly OK.
Nobody can decide for you or know if the seller you indicate is honest or isn't.
While there is always a risk buying from someone you don't know, and buying from B&H has no risk at all, I myself took the risk of trusting the eBay customer protection if something goes wrong, and my seller was luckily indeed a honest person.
I saved not a fortune(!) but in percentage quite an high percentage of money as compared to the official price Apple asks over here for the AppleCare of a nMP without finally any difference at all in my coverage. I was therefore lucky.
Decide by yourself.
I can only tell my own experience.
In MY PARTICULAR CASE it worked fine.
P.S. I forgot to mention that Apple asked me to mail them my purchase invoice of my nMP and next day they informed me that my AppleCare had been accepted.
Probably if I had bought the AppleCare together with the nMP (at the quite higher Apple official price) this step would not be necessary.
Anyways it was no problem to follow Apple's instructions and the final result just took 1 day longer...