As long as we're talking about purely hypothetical situations, not involving in any actual people, how about this one:
A person lives in a state in which handgun sales between individuals are legal without any state registration/reporting system. This unfortunate soul is moving to Massachusetts and would like to get several nice guns while he or she is still able. Since the cost of this collection greatly exceeds the cash currently available he/she decides to purchase them on what might accurately be described as the lay-away plan. A friend purchases the desired guns, then gives the poor migrant a signed and dated bill-of sale for each gun. When the new victim of the PRofMA manages to scrape together sufficient funds, he/she takes a visit to the original state, pays the friend and takes possession of the guns. The friend, of course has been charging a handling fee on the transactions by, well, handling (and shooting) the guns.
It's hardly a scenario I'd want to ask the authorities about ahead of time, or admit to publically afterwards, but all sounds completely (if only marginally) to me. Even if I were totally wrong and it were somehow deemed to be illegal, how could anyone hope to prove it actually happened? The guns were legally purchased from a licensed dealer in the home state prior to the move, and the new Mass resident has signed bills of sale dated when he/she was a legal resident of the former state of residence. Then again, it's only a hypothetical situation as we stipulated originally.
Ken