Let's say you purchase a AR type stripped receiver from Joe Blow at a gun show. He claimed it to be never built, and you log it in as a receiver.  You sell said receiver to Herby Handcock, as a receiver, who builds an AR pistol. Well Herby gets into some trouble and some ATF agent researches the receiver and finds out it left the factory as a rifle. ATF tracks down Joe who is a LEO and claims he sold it to you, the dealer, as a complete rifle. ATF tracks you down and you claim it was a receiver and your A&D book and 4473 reflect that.  Who, if anyone will the hammer smack?  
It's not required by ATF, but to CMA and for accounting reasons, I require a signed detailed BOS for all firearms and serial numbered accessories.

Nobody as none of the parties knowingly or intentionally broke the law.

This,
Nothing illegal occurred.

Scratching head on this one.

I buy 2 virgin receivers.  Build them both as rifles.  I sell the uppers, stocks, & parts kits, give one receiver to my brother.  He knows it started as a rifle, but he builds a pistol anyway.  The other stripped receiver I sell here on the EE & falsely claim it's virgin.  The ARFCOMmer also builds a pistol.  Did my brother break the law?  Did the ARFCOMmer?

So once discovered that the receiver/pistol was originally a rifle, wouldn't it have to be disassembled?

In this case there is no record of them actually being built so it cant really be proven.

And if I'd posted my twin rifle build piece by piece in glowing color on ARFCOM?  Where is the bright line?

A stripped lower can be built into a rifle and later into a pistol.
A pistol can be built to a rifle and later a pistol also.

No but thanks for playing.

Got a cite for that?

If I'm not mistaken, legally, only on a Form 1.

Well then you screwed yourself.

Err...I meant the other way around.

A stripped lower can be built into a pistol and later into a rifle and back to a pistol.