In EVERY case they were thanked for their time and NOTHING happened.
That's how I've seen it as well. Let's say I sell a gun to Barney in good faith, and with no reason to believe anything illegal will come of it. Barney needs money, and six months later he sells the gun to Pete. Pete uses it for something nefarious.
If the ATF, the local prosecutor, or anyone else wants to blame me for that, they can get a warrant and arrest me. They'll have to
prove that I knew something bad would happen when I sold the gun. Absent that, I can't be charged with anything.
I did once have a situation with a gun I sold. I traded it to a retired LEO, who traded it to someone else a few years later. A few years further down the road, something happened, and it ended up being confiscated by the DEA in a drug bust.
They traced the gun to the retail point of purchase and my name was on the 4473. They sent me a letter.
(I had to sign for a certified letter from the DEA. The mail lady was glaring at me the whole time.)
The letter was a notice that the gun had been seized as per whatever the law is on asset forfeiture, and it stated that I'd have to prove ownership and go to court if I wanted it back.
I ended up calling the office to clarify that I hadn't been in possession or control of the weapon in years, and that I had a signed bill of sale...and...and...
Oh. What? OK. No, I don't expect to get the gun back. I was paid for it. It's not mine. So, that's it? Oh. OK. You have a nice day, too.
That's the last I heard of whatever happened there.