An interesting idea and I wish you the best of luck.
But... Its always great to read someone complementing you and then following it up with a "but".
But it does sound a little like a solution looking for a problem.
If a camera/lens is found by a non-photographer (let's face it, most people fall into that category), they won't know to even access the lenstag website, unless there is a sticker on the camera "if found, go to lenstag". They will turn it in to the police or keep it, negating any benefit of lenstag. The police, unfortunately, do not have the time nor budget to actively try to find the owner of lost property; but wait for the owner to contact them. I doubt that any police department will contact lenstag, even if they knew of its existence.
If someone is interested in buying a lens cheap through a private buyer (craigslist), it does them no good to check with lenstag after the sale. So they would have to check with lenstag before the sale. This presumes that the craiglist type buyer 1) cares if the lens is stolen 2) knows about lenstag, 3) is able to get the serial number prior to the sale from the criminal seller.
Let's suppose this thing works out. I am a lenstag customer. I report one of my lens as being lost/stolen. A potential buyer, somehow, checks with lenstag and discovers that the lens is reported lost/stolen. Lets assume that this buyer does not just leave but reports it to lenstag. What happens next?
I assume that lenstag sends me an E-mail (since that is all that lenstag records) and tells me exactly what?
That some guy reported that some other guy is trying to sell my lens? Even if the seller's identification is given to me, am I supposed to go all OJ Simpson and try to recover my lost property?
Or am I supposed to call the police and tell them that I got an E-mail from some website that says that some guy knows some other guy who is trying to sell my lens. And the police do what? This is a fourth hand tip.
Does lenstag record the potential buyer's name and address (assuming that the buyer even left his or her name)?
Your website allows anonymous tips. How valuable would that be when there is no way to follow up with questions or to allow the police to talk to the person making the tip? Have you considered how this could be misused/abused. Its the Internets Tubes after all.
Assuming that it is a very very slow day at the police department and they send an officer out to talk to the seller. The criminal seller tells the police that, yes, they sold "a" lens, but they can't remember the serial number (there is no requirement for anyone to record serial numbers of camera equipment when privately sold); nor is a private seller obligated to keep or show any record of the sale to the police.
Pawnshops might be interested in this, but they also get lists from the police. I am not sure how many stolen photographic equipment is sold to pawn shops, so it might be a good customer base. The problem will be getting pawnshops to 1) know about lenstag and 2) voluntarily use it. Both of those are completely out of your control.
I think your idea of lenstag has a good premise, but I think you need to think about the details of what really your service can and can't do. From what I read on your website, a lot of things completely out of your control have to work just right in order to have any success. Your system would work, if enough people voluntarily participates in it.
One of the advantages of your service is that you don't charge for it...yet. Which leads to the usual types of questions about "free" "stuff" on the internet. Are you selling (or allowing access to) E-mail addresses to marketing companies? I am sure they will pay well for my e-mail and a list of what types of expensive photographic equipment I have. My advance apologizes for my paranoia, but I have stopped trusting people on the Internets Tubes a long time ago. Often, when the product is free, it is the customer's data that's the product. Perhaps you are the exception.
I think a better solution is for the owner to have insurance; have the owner keep records of serial numbers and pictures of the equipment; and then contact the local police stations if their equipment is lost; and if it is stolen file a police report.
Good luck with it, but I think you have some more work to do.