AppleCare registration is usually done over the web. I don't know if Apple has regional keys (i.e. certain numbers are sold in certain regions), but so far it hasn't mattered much. Of all the Macs I've owned, I only bought it at the Apple Store once -- that was at full price. I've since learned other stores, including Amazon and LACC, sell it for far less.
Regarding eBay, the usual rules apply: make sure to deal with a reputable seller (check and read ratings) and pay with a Credit Card for additional protection (or use PayPal protection if available, though I don't really like dealing with eBay/PayPal).