Keep in mind that keys from eBay are usually for another region and/or may be bought with stolen credit cards and resold through eBay (and, if the publishers decide they don't like you buying for another region or the keys are fraudulent, they will be removed from your account).
Just putting it out there as a warning; just know what you're getting into. Side note: reports say that, most of the time, they just deactivate the key, not disable the entire the account (if that's your worry). Proceed with caution, but it's your choice after you know what you're getting into.