are you seriously expecting people to just be on the 'lookout' for other peoples cars and take down number plates and then double check?
every car that you see could potentially be stolen, and every stolen car has a thousand identical ones running around...
its a nice idea, but cant see it being much use unfortunately...not without a proper searchable database etc
the only use i can see would be when buying a second hand car, you can check to see if its listed on there...but then there are other better ways to get that info