I think you are missing the detail of his post.  It looks like he just sold the barrel, not a barrel and reciever.  
Nope I understood that.  A shotgun barrel by itself under 18" requires paperwork as class 3. 
Interesting, I was under the impression that you could possess a barrel <18" as long as you did not posses a shotgun receiver.  I was not aware that posessing a barrel alone in and of itself would go against ATF regs.  
You can buy short barreled AR uppers from just about any shop and they are not a registered weapon by themselves.  As long as one does not posses an AR reciever at the same time, there are no ATF regs that are being violated that I am aware of.  Maybe shotguns are treated differently or I could just be misinformed.
The only SBR I have is a registered AMD65 so I don't have that much experience with class 3 stuff.