Most common thing I see are stripped barreled actions, being sold as "C&R eligible"- and that's patently false. They need to go to an FFL, period- but I guarantee you can go to that auction site any day and find a half dozen listings like that.
Most if not all of the major C&R vendors have sold barreled actions as C&R. Off the top of my head, Century, SAMCO, R-Guns and SOG have all sold barreled actions as C&R, and they get audited much more often than you average C&R holder.
In fact, Samco has a few listed right now:
https://www.samcoglobal.com/1-swed-SM38.html
https://www.samcoglobal.com/1-M24-47.html
on edit: SOG still lists Enfield Jungle carbine barreled actions, but are out of stock, and you need to be logged in to see it.
Here is the Wayback Machine copy of the Rguns bolt action rifle page from 2011, showing both Mosin 91/30 and K98 barreled actions as C&R.
https://web.archive.org/web/20110505085955/http://rguns.net/rifles/rifles-bolt.shtml
I bought one of the K98 barreled actions, and turned it into this:
Samco has been selling them for years (since at least 1997
according to the Wayback machine), and Rguns had the RC K98 Barreled actions for several years (according to the Wayback machine from at least
Dec 2008 to
Nov 2012) before they finally ran out.
The argument could made, that an otherwise intact barreled action is unmodified, just missing parts.
If you take the view that a barreled action is not C&R, how many parts are allowed to be missing before a gun loses C&R status? The stacking swivel screw fell out of my M1, and the swivel fell off, did it lose C&R status?
Just about every vendor has sold "Gunsmith Specials" with missing parts. I don't see a barreled action as any different, provided what you get is in the original configuration.
The "Original Configuration" thing is really the poster child for executive overreach. It is not mentioned in the regulations, it is a ruling ATF made (ATF-85-10), in response to a law passed which they didn't like (requiring them to allow importation of C&Rs), which they tried to limit as much as they possibly could.
They then applied it globally, which in my layman's opinion is not reflected in the regulations. The regs says a C&R must be one of three things to qualify. ATF says,"never mind that, a C&R is what we say it is".
The fact remains, pretty much all the big C&R vendors have sold (or as noted by Samco, currently sell) otherwise unmodified barreled actions or you-fix-em/gunsmith specials as C&R eligible.
Do you think they would do that if ATF didn't think it was legal?
Of course none of that applies to the original question. A bare receiver is still a firearm, and the seller is still an idiot.