First make sure you're not inadvertently substituting a lowercase l (L) for an uppercase I or a zero for an uppercase O (or vice versa).
Secondly, the serial number stickers were affixed to the front* of the the booklet titled "Installing Your Software" which should have been included in the paperwork with your purchase in order for it to be a legal transfer of the license.
And finally, Apple does have a web page regarding serial numbers, however, it requires proof of purchase and I don't think ebay purchases would apply. Wouldn't hurt to check though. http://support.apple.com/kb/HT1861
*Note - with FCS3 (FCP 7), the serial number was on the back of the booklet.