i mean, you can make a gun out of anything really, from godawful pipe, nail and a plank of wood shotguns to the sten submachinegun
the 3d printer just makes it easier, and less skill/labour intensive
the problem will be ammunition, if you are going to build something that fires shotgun shells or rifle rounds, you could get them from a gunshop if you had a license, but anything that fires pistol calibre rounds, you would have to buy those black market
with the trade thing, there already are illegal workshops churning them out, mostly (from what I remember) recoil operated submachineguns, you can make them pretty easily from plans on the internet with a cnc lathe