It is possible to get burned, though it is rare. I bought an Air months back from an eBay seller, new, with no original receipt. Apple talked with me on the phone and did everything they could to shrug me off, finally using "no original receipt" as the excuse to not accept my claim.
This was for a torn layer in my LED display on the Air. If it's something they consider to be cosmetic...like a giant ***** blob in the middle of the screen...they'll do what they can to avoid a warranty claim.
Like I said, it's rare, but it can happen. When you register your computer online and it says to save the receipt, they are serious. Of course, stroke as a side effect of taking birth control pills is less than 1%, yet I know somebody to whom it happened.