Is it safe to buy a MacBook Pro from some guy on eBay?
This is a question with an impossible answer. Use the age-old adage "if it seems to good to be true, it is."

For the most part, eBay is a mixed bag. It really depends how you shop and what kind of items you are in the market for. Personally, I've been burned far more than I've actually saved and no longer use the service. I will likely give you a biased answer and if pressed, say "no." Unless it's a private seller (which likely won't part with their machine for "substantially" less), I would be wary. Apple maintains strict pricing on their products, which is why only marginal price savings can be found.

As for where do most people get theses Macs; if they are selling them for substantially less, then you likely won't get a genuine product, or they have likely acquired them illegally.

AppleCare must be purchased and registered through Apple. No other company can provide you with that kind of coverage. Period. Discounts on the coverage (as with select Apple devices) can be had using their educational discount, which applies to students. And to qualify, the product and the plan must be purchased through their Education Store.