Surely they have some bots that can do that kind of thing for them, or at least turn up specific things for somebody to check into later.
I just wouldn't want to underestimate the lengths to which the government will go to screw over a citizen.
In my experience, getting the US attorneys and/or ATF involved with one-guy-one-gun where you serve it on a silver platter as a completed investigation isn't easy. Convicted dope peddler shooting at a rival dope peddler with an SKS? No, state level charges only. Felon in possesion who fires through an apartment and hits someone in another apartment in the spine? Nope, state level charges only.
Ask the folks who run you local gun store how many people actually get prosecuted for lying on the 4473 after a turn down.
Based on that, I'm thinking they aren't busy running down random jackwagons who screwed a ZIP to an AR on armslist.
I think you overestimate the manpower and resources of the ATF. Getting them interested in one-guy-one-gun cases isn't easy, even if the one guy is a violent gang member. I'm thinking trolling through a gun forum waiting for someone to post about a ZIP on an AR isn't really something they are in to.