Every sale to every individual buyer requires separate handwritten notice, each individually attached to a copy of the privacy policy and the data sold, notarized and sent by certified mail in triplicate, with postage paid by the sender. Make it cost so much that the entire industry becomes obsolete.
Most currencies have a special pattern that printers are programmed to detect and refuse to print. Since illegal gun part designs can’t be forced to include a marker declaring that they’re gun parts, a 3d printer would have to 1) know what a gun is, 2) know how a gun works, 3) be able to tell whether any particular shape could be used as part of a gun, and 4) be able to tell whether any particular shape could be cut and reassembled into a shape that could be used as part of a gun