Thingiverse itself doesn’t sell anything. It’s a repository for digital plans to built stuff. It’s also not limited to 3D printers, but it’s a place for digital plans for physical objects. (The same parent company does sell 3D printer kits though.)
That being said, here’s a line worth immortalizing from Bre Pettis: “Recently there has been a lot of discussion around guns since the lower arm of an AR-15 model went up on Thingiverse.” I have to give him credit though, nowhere does he say “clip” when he means “magazine”.
What someone needs to do is to upload a Sten Gun template, all ready to print out and wrap around a piece of pipe. One of the owners just might have a shock absorber with the right size tubing, thereby creating “constructive possession” of a machine gun (j/k