While I agree it would be unwise to test this legal question in court, I think it is fair to say this is a murky legal question. The BATFE, the federal law enforcement agency responsible for enforcing US laws regarding firearms manufacture, have a history of arbitrary rulemaking in defiance of simple logic (for instance, in one famous rulemaking BATFE ruled that a shoe string would meet the legal definition of 'machine gun' if used to repeatedly fire a gun with a single pull of the trigger [1]).

Given this environment of legal uncertainty, it's not at all clear that BATFE would not pursue the 3D printer operator who produced the receiver. In this case, charges against the 3D printer operator would likely be unlawful manufacture of a firearm, possibly in addition to charges related to interstate transfer of a firearm. Given the creativity of some US attorneys, perhaps a conspiracy case would be made alleging the buyer of the printed part 'conspired' with the printer operator.

I would also suggest a lower receiver could probably be made from materials weaker than you might imagine. The lower receiver does not bear any gas pressures from the firing of the cartridge, though the holes for the roll pins which hold the hammer and trigger in place would probably start to 'egg' after a round or two. In any case, the legal question has nothing to do with how reliable the resulting firearm is; only whether it meets certain vague and broadly-interpreted legal definitions.

[1] http://www.everydaynodaysoff.com/2010/01/25/shoestring-machi...