Well here's your problem, if you made model which resembles their product and given away for free, they are probably not going to say anything about it. But if you actually sell them for commercial purpose (which is the keyword) without informing them first or made a contract with them regarding its designes, sooner or later you will be served with a court order.

I've seen lots of free firearms models in the freebies section and they look exactly like Heckler & Koch firearms, it's been there for years and so far they have not been taken down due to manufacturer's complaint.