More and more Apple is moving to an auto-enroll style of AppleCare, where it is tied to the machine at the time of purchase. That said, retail packs are still made available at authorized retail stores and other places. Your guess is as good as mine if the trend for more auto-enroll will continue replacing the box version.

It sounds extremely fishy. All Apple products have basically fixed prices. If anything is sold new at a significantly reduced price, particularly if they're not an authorized retailer, it's most likely at least gray market and could be somehow fraudulent. An incremental discount could be the result of someone abusing the educational or other discount programs offered by Apple. In a case like that, an audit trail will exist, and while purchases like this might work at first the plan could be cancelled later if it's determined that it was activated outside of the Ts and Cs of the discount buying programs.

Not worth the risk IMO, and I have a pretty good vantage point upon which to base this assessment.