Amigo, if the seller makes a living selling Apple products, their reputation as a vender on Ebay is completely relevant. I have been buying Apple Care from Ebay venders for over five years for iMacs, Mac minis, iPods, iTouches, iPhones, etc. and have successfully used the Apple Care for warranty repairs and replacement. I have never been shipped packaging. I have always used a vender that emails me the serial number.
If an Apple Care serial number was not any good you would know that as soon as you tried to register the Apple product with the Apple Care serial number, not some period of time down the road when you needed to rely on it for coverage. Apple does not work that way as a corporation.
If you are really gun shy about using an Ebay vender, it is probably best for your peace of mind to buy from another source.
Dah•veed