It's pure fucking dumbassery even to end up in this situation. Why would anyone sell a sawed-off shotgun? Just sell them an actual shotgun, and point out it would be completely illegal, but yanno, I've heard some people, criminals, yanno, will sometimes saw off the barrel right here. Now you'd best not do that, because it would be illegal, but bad people do that sometimes.
I mean, if you're a guy who moved out to the middle of the rockies in montana to just get away from it all with the family, a guy asking you to do some gun smithing and shorten some shotgun barrels would probably be a wink and nod sort of deal. I doubt anyone aside the agent arranging it all could even comprehend the kind of response they got.