If they want to monitor sales of 3D printers because it can allegedly be used for manufacturing of firearms replicas, then they have to monitor sales of every other hand tool: drills, files, saws... anything
Not to mention lathes, mills..
Almost all 3D printers for home are using plastic as material. There are 3D printers that use metal (titanium for example.... I saw demonstrations of couple of models at CSIRO facilities in Clayton last year) but they are very expensive equipment.. and even their printout is not much more precise compared to my home made 3D printer (I can do +-0.15mm if I am lucky), which is good for many things but totally insufficient for manufacturing the working firearm ( I am guessing the tolerances there should be around 0.005mm max). I simply don't believe plastic gun (without metal parts) can fire a shell for the start.
Someone can 3D print a handle for example... but then again almost anyone can make a better and stronger handle from hardwood, at home, using only saw, file and drill.
Any mechanical metal work shop (with manual tools and lathe) can do way better, both in terms of tolerances and mechanical strength... not to mention buying real illegal arms from dealers.
So all this is barking up the wrong tree... IMHO.
The only concern regarding the home 3D printing are replicas which do
not fire, as was demonstrated by the most recent case in Sydney.