People collect guns, then sell them. People move into the state with them, among other things. There are also a lot of LEOs who collect guns and then turn stuff over that they don't want. Used 3rd gen pre cutoff glocks naturally find their way to MA as MA is basically the only state with this dumb regulation on dealer sales of Glock handguns. The economy sucks, so often more people right now are selling than are buying. It's not rocket science- no "straw purchases" or other illegal activities required.
Do we really need another MA compliance thread in GD?