I'm willing to go the non-conspiracy route, but do not like the idea of the IRS getting involved in 4473s. Before you know it, the EPA will be asking for them so they can do searches for "guns that shoot lead bullets". The ATF making lists is bad enough.
Also, and it needs to be confirmed - if the IRS took ONLY 4473s, then what are they cross-referencing with?
I assume the seller fills out some IRS form that says some version of "Isold x pistols for $zzz, and y rifles for $aaa, and my excise tax owed is $ccc"
Probably quarterly?, when they pay payroll taxes and the like.
Also, just in case it's not explicit from my other posts, I agree I don't like the IRS involved. The whole agency can get sucked into a portal to Hel and I would rejoice.
I'm only pointing out that there's a pretty obvious reason the IRS might want to know how many and what kind of guns a place sold, and it's unlikely to be a prelude to door to door checks.