Skans, I am opposed to making any such thing mandatory.
As an optional thing, on those occasions where I sell a gun to a stranger, for instance via GunBroker, I choose to conduct transfers through an FFL.
For people I know, who live in my state, I don't use an FFL.
(Note: I am not in the business of selling guns, but with a baby on the way, I've sold a few, and with a full safe, if I want a new one, I sell an old one.)
But I am firmly in the camp that believes if the government can force "Universal Background Checks" down our throats, a federal registry won't be far in the future.