Realistically, if they track usage of NICS then they know every time you buy a gun from an FFL, and which FFL you bought it from anyway.
They can come to that FFL and audit their 4473's any time, so, if they want a list of all guns you've bought (from other than face to face private in-state transactions), they run a query for all NICS checks on you, pull up the list that shows your info and the place that requested each check, then send out someone to check the 4473's at each location. An hour later, they have a compiled list of every gun you've bought, from which locations, and when.
...and people say a national gun registry doesn't exist... just because it's not a centralized, digitized database containing all information needed doesn't mean it doesn't exist.
Anyone know if usage of the NICS system is actually not logged/tracked? Heh. It seems to me like they'd have to log it if for nothing more than billing the FFL's...