I'm sure that the law is only enforced when and if they get around to it, and it most likely is more closely monitored for people that are producing unregistered weapons for sale or distribution, but the fact that I would be breaking the law and could face federal charges and huge fines would definitely deter me from infringing on it for a handfull of one-off parts. I was contacted by them for developing a GPS guided model rocket. I hadn't even considered producing them or selling them, it was just a project I was brainstorming with another guy for his remote controled model submarine. It probably didn't help matters that the guy was from sweden. So yeah, they do keep an eye out... If you aren't registered, then you shouldn't sell anything over the internet, that's for sure.