since a mixture of diesel fuel and fertilizer is way the hell more dangerous than a gun could ever be.
Note that post Oklahoma City - large purchases of fertilizer and other unsophisticated bomb ingredients are, in fact tracked much more effectively than gun purchases.
We're currently entering a phase wherein the development of "big databases in the sky" (BDBS) are easier than ever. We've also entered a phase where contributing to the BDBS via portable hand-held electronics (smartphones) or wearables (Google Glass) is getting even easier. The crux of the problem here in the states revolves around issues of privacy (and the right thereto), bearing arms (and the right thereto), and free speech (...). Where does one right end and the other begin?
If it's private citizens participating in a BDBS - can your right to privacy said to have been violated? What about their right to free speech? And they have definitely not infringed on your right to bear arms (in the aforementioned case). If this were a government backed initiative - there would be problems galore, but it's not. Since you trotted out the straw man of the sex offenders - lets remember that's a government initiative, so that makes it different in means, if not the ends. One could certainly argue that such registries constitute cruel and unusual punishment (I'm not, but for the sake of argument). If we're afeared of sex offenders, why not have people convicted of DUI have a "scarlet letter" on their license plates?
I think we all see where this ends. If the government is engaged in this - it's automatically "bad". If private citizens build and offer such a service, is it automatically "bad"? In either case the ends are the same - there's now a publicly available database of information on you - which you don't have control over. How do you deal with misreported or erroneous data?
What about a Slashdot like system of moderation? Upmods and Downmods. Karma, etc.? Build a HUD into new cars, and you can autotag the dangerous drivers - and boom, up to the BDBS! Get cut-off, notice a speeder, or an erratic driver - and you could report it to the BDBS. That info would be available to your HUD, and automatically overlay onto the other drivers on the road in your field of view. You could steer clear, literally of bad drivers. But what to do about those who report everyone but themselves as bad drivers? Karma. The downside of this is we become our own surveillance state. The upside is that police could concentrate on real crime - you'd just get your tickets and auto insurance hikes in the mail.... enough of them and your drivers license gets auto-suspended.
The same sort of thing could be said about guns, and any number of other activities we engage in. I'm not sure that's a world I want to live in. But I'd also say a few reads of David Brin on privacy and how some aspects of how it's evolving might be food for thought regarding the privacy and/or transparency of things.