Partially completed receivers are considered "munitions" under ITAR if they "have reached a stage in manufacture where they are clearly identifiable" as a firearm receiver. I'd guess that an 80% receiver is already "clearly identifiable" as a firearm receiver, but don't know for sure what the State Department's opinion is.
Articles on the U.S. Munitions List include articles in a partially completed state (such as forgings, castings, extrusions and machined bodies) which have reached a stage in manufacture where they are clearly identifiable as defense articles. If the end-item is an article on the U.S. Munitions List (including components, accessories, attachments and parts as defined in § 121.8), then the particular forging, casting, extrusion, machined body, etc., is considered a defense article subject to the controls of this subchapter, except for such items as are in normal commercial use.
How many were you looking to get?
Plenty of blueprints out there. You could check with some local machine shops and see if they can do a run for you, which allows some profit to be made on your end.
ETA: I believe there is a Chinese company (Wo-Tech) which exports 80% receivers. Maybe buy 1 or 2, for QC, before buying a bunch?