CBI Checks, aren't they supposed to be purged?

[Great-Kazoo]

FFL holds on to them. if FFL sells the business or does not renew his ffl, off the forms go to ATF[e]

As mentioned above. Serial number & mfg info allows feds to contact mfg, who then directs them to vendor or ffl . Once ffl is contacted the 4473 search begins. That is how they know who purchased what & for how much. Not through CBI but just plain old police work.