From a previous post ...
"I guess I should elaborate. I purchased AppleCare from a "PowerSeller" on eBay for $200 for one of our 17" MBP on March 2008. I received a sealed AppleCare box and registration on the Apple website went well. About two weeks later I received the official AppleCare letter from Apple validating the coverage.
That 17" MBP had to go through 2 battery replacements, 2 screen replacements, a logic board replacement for the Ethernet port, and a logic board replacement for the nvidia 8600. The last problem we had was when the screen they replaced developed weird colors for the bottom 1/3 of the screen. At that point it was agreed they would replace it.
They send a new 17" MBP. Seeing as I had purchased it in 2008 I had plenty of AppleCare coverage left, so they said I could cancel AppleCare and use the credit towards the purchase of a new plan. I sent all my information to the department I was told to but once they saw it was purchased on eBay they said that the original AppleCare was not valid.
On one hand they serviced the original machine plenty of times with the AppleCare coverage and they ended up replacing the MBP with a new one because they couldn't fix their mistakes. So I am happy I have a new MBP that is working fine but I'm disappointed because it now means I have to shell out $349 to buy AppleCare from them directly as I have a feeling they will check the validity of the AppleCare plan if I have any issues with the new MBP. So I essentially paid $200 (ebay AppleCare) so they would correct their mistakes and replace the machine.
Having said that, I'm sure AppleCare purchased from eBay will be fine for most warranty work performed by their service centers but when it comes to "big" issues you might have some trouble."