My understanding is that in this case, the seller did not submit proper paperwork to export these gun parts to the US. The part may very well have been perfectly legal to own, except that the paperwork had not been done thereby making the parts illegal to own. The buyer had no knowledge that the seller had failed to conduct his business in compliance with the law, that's why he's still walking the street and is in no legal trouble. If you buy your parts from within the US, chances are the original importer (the wholesaler) had obtained the proper paperwork. Hence, there's nothing more you need to do. For internet sales, the only burden you're assuming as a buyer has to do with whether or not these parts are legal to own in your area.
This particular seller must have done something highly suspicious (or even illegal) to get the ATF's attention in this. I doubt they would have done anything over a few threaded barrels and so forth. Now, if unregistered silencers, automatic weapon parts, and the like had made it in, that would get their attention in a heartbeat.
