I'll talk from my personal experience.
I bought the Aluminum Macbook 2.8 in October 2008 when they were released. I believed (and still believe) an extended Applecare warranty was (and is) potentially very useful (based a faulty logic board in my 2002 iBook and a defective Ram slot in my 2005 Powerbook 15").
I purchased my Applecare online from a reputable seller with 1000+ feedback and almost all positive.
After the purchase, I applied my Applecare and confirmed it was valid.
In September 2009, I checked my Applecare status. It stated that I had not purchased Applecare.
I tried contacting the eBay seller with no avail. Paypal/Ebay could not do anything. I consider this a $100 learning experience.
This might be a $135 learning experience to the OP. But even that is cheap in comparison to much larger, more costly learning experiences in the future.
Here is my experience:
I bought a late 2008 unibody 2.4 ghz MBP in January 2009. Few months later I bought AppleCare from eBay (serial via mail only) only later to realize what kind of risk I took with this purchase.
Fast forward a year and few months (its april I think, 2010) I have 4 repairs behind me (display, logic board, fans, HDD) and apple warranty still valid. I contacted apple, complained about so many repairs, and they replace my 2.4 C2D with a brand new 2.53 i5.
Now here is the catch: apple offered to transfer my remaining apple care days on the new MBP. The thing is, AppleCare ticks from the purchase date. So, I had about 20 months left of applecare from eBay, but if I were to transfer it to my new machine, it would tick from the "purchase" date, meaning I'd have AppleCare covering my 1y warranty as well. This meant I'd have less than a year warranty extension outside my standard 1yr warranty period.
As an alternative, apple offered to return the AppleCare purchase money to me. I thought: "cool, you payed 150*on ebay for AppleCare, and apple will give you 200+*back (they don't give out full amount if you had a repair)". But the thing here was, they wanted to have a receipt of my AppleCare.
Since I had none, I said that I didn't want the money and I didn't want them to transfer the remaining AppleCare days either, as that would leave me with not even 2yr warranty MBP.
In the end, I got a brand new 2000*i5 MacBook Pro from my eBay purchase, but I will not purchase AppleCare on eBay again. I'm gonna get it from Apple directly this time and be on the safe side. I consider myself REALLY lucky on this one and I don't recommend anyone buying AppleCare on ebay (unless you are getting the box).
