One of the great things about the internet is the ability to deal with information that is far too diffuse and voluminous to be processed in other ways. Indeed, that is the principal way in which modern computing qualitatively changes that we are able to do, as opposed to altering the rate at which we can complete a particular task.
Given those characteristics, it surprises me that nobody has come up with a site that catalogs serial numbers for all the kinds of products that include them: from bicycles to cameras to mobile phones. Such a site would allow users to enter that information when they purchased a product. It would then be on hand for warranty claims and in the event of loss or theft. People purchasing such items online, or in used good shops, could check the database to ensure that the products they are buying are not listed as stolen. Like eBay, it is much more efficient to have all these numbers sorted in a single place than to have numerous separate databases. The chances of a person trawling through many sites are low, but one well organized one could get masses of traffic. (See: network effect)
You could even imagine a system where online retailers like eBay are integrated with such a site. The listing for a camera would thus include a serial number linked to an entry in the database. If you bought the item, then received one with a different serial number from the one listed, you would be entitled to lodge a complaint and the seller would get flagged as a potential fraudster. I have personally avoided buying photographic equipment from eBay because I fear that a lot of it may be stolen. Having some simple protections like these in place would make me feel a lot better about it.
PS. For an example of an existing but limited serial number listing, see the stolen equipment registry over at Photo.net. It is unlikely that someone buying a cheap digital camera online will look at that (I knew it existed and it took me some searching around to find the URL), but perhaps someone buying an expensive tilt-shift lens for a medium format camera system will.