One primary question; who could verify? Who could vouch for other people, could I get a buddy to come on here and say "Yeah, sure he's licensed to own full auto." so I can buy a machine gun?
The problem with a system like this is the burden of proof, unless we provide the verifier with hard copies of our licenses that are thereafter overseen by a certified and neutral third party, then there's no real proof that someone actually gave proof that he is licensed. It is open to too much word of mouth.
How many times have you actually seen the PAL of people at your range, or who you know own guns? You might go hunting with a guy every year for twenty years, and vouch for him being a great guy when he needs a reference, and then you realize he has never been licensed. Without hard proof of proper licensing and third party verification all it takes is one guy and his friends to get in on it and get caught with a gun they bought from us and then the next day in the news we all hear about the "Nation-wide firearms trafficking ring exposed!" And the problem with hard proof third party verification is that it creates another point of entry for the system to be compromised, much as the registry did. Someone would access the records and then suddenly copies of all our PALs are bouncing around the internet.
Sad, but that's it is with today's society and system and the enforcement of victimless crimes.