they (the ATF) inspect your "bound book" in which all transactions must be recorded. As a small dealer selling only 50 to 75 guns a year I can attest that it is easy to make a error in the book. The ATF will go back to the manufacturer of a gun used in a crime and look to whom it was shipped. You had better have your I's dotted and your T's crossed as the ATF will not have mercy on you.
FWIW, this is true for any regulated activity....OSHA, MSHA, EPA, DNR, Sarbanes-Oxley, NPDES, ATF on quarry blasting, etc. (who is etc?)
This from the agency that considers a part/componant with a SN number on it as a firearm. Doesn't matter that without a bunch of other parts/componants assembled correctly it won't fire (the main part of firearm) and is only suited for a paper weight.
Also, the agency that does not know how to properly test firearms, claims a shoestring is a machine gun and had NO standardized written procedures for determining whether a firearm was legal.
What we need is a President/Congress that will beat the snot out of them, if not eliminate them completely.