https://www.cnn.com/2019/10/11/us/ar-15-guns-law-atf-invs/index.html


Very lengthy story, so I will not add the text. 

But long story short..... dude was selling home-made AR's without serial number (which is illegal)
Building your own AR without a serial number is legal, but you cannot sell or transfer.

Defense brings into question the ATF definition of firearm, which currently includes the AR15 lower receiver.  According to ATF definition, they realize the AR15 lower receiver does not technically meet the ATF definition of receiver or firearm. Judge rules as such. He's right.

ATF backs off the prosecution, fearing a decision would create a precedence declassifying AR15 lowers as firearms.

Such a long article, I don't know if he was selling the entire rifle or just the 80% lower after completion.