Just speaking from practical experience here, not legal expertise:
The way this is handled in terms of shipping is that the sellers put the burden on the buyers. Dealers who are concerned with the federal reg's simply require buyers to authorize a disclaimer saying that the buyer qualifies for the federal exemptions. That would be how 99.9% of us buy autos.
Arguing over whether the knife has *ever* crossed state lines seems like kind of a fine point to me. Seems like the issue is whether it crosses state lines in the course of one's own procurement. IOW... I live in OR. Autos are completely legal in my state, county and city, as long as they are not carried concealed. So presumably I could buy all the BM's I wanted from the factory or an in-state dealer, and that would be 100% kosher. The way I look at it, I could also buy all the MT's I wanted from in-state dealers. No in-state dealer has ever asked me for any identification of any sort.
Yes, technically it is illegal for me to buy autos from out of state dealers - whether they are shipped to me or I carry them home myself. I really think you'd have to be pretty uptight to worry about this at the personal consumer level, though. Certainly understand there being concerns at the corporate level, but not for a typical civilian in Kansas who wants to mail order an MT from TAD.
I mean, it's a federal infraction to swap your exhaust if the OE cat is in working order, even if the new exhaust passes DEQ. But only whacko states like CA get caught up in that... and a quick jaunt down I-5 reveals that even the Californians don't take this too seriously. I can drive around all over OR with no exhaust at all. As long as I stay out of the metro areas and dont make a fool of myself, no local/county/state cop is going to say squat and I can assure you that Im not going to be pulled over by the FBI. Lol.
I really think the Federal authorities have better things to do than bust a guy for buying one, or 5, or 10 knives with springs in them. Producing hundreds of thousands and distributing them to the Girl Scouts, yes - but not just picking one up on vacation.