This!! IMO you always have to conduct business as if you are dealing with a LEO at all times.
PRECISELY.
I actually had a uniformed IL State Police trooper sitting at my kitchen table once (after waiting the waiting period), buying some AR receivers I had left over from when I had my FFL.
He got a little irritated when I asked him to sign a paper stating he was the actual buyer and not a prohibited person. "The law doesn't require THAT."
"Yeah, but *I* do, and I don't care if you are God himself, you sign the paper or you don't get the receivers."
I was actually happy to sell the receivers to the police - At least I know they won't end up in the hands of a gangbanger.
Between having the FFL, doing gun shows, and making what few private sales I've done from my personal collection, I'm sure at SOME point I sold a gun to an undercover cop (one in particular stuck out where a guy was really insistent on picking up his gun the next afternoon - "I've waited a day", but a full 24 hours hadn't elapsed yet. I made him wait around until the clock read the right time, per IL law.)
I always assumed each sale is to an undercover cop, so I don't cut any corners. Kept me on my toes making sure the paperwork / etc was correct.