You can sell guns, as long as they were manufactured before 1895, they are exempt from federal firearms laws.
You can sell parts of guns, for the AR-15 you can sell all parts but one, the lower reciever, which I can print out on my 3D printer.
There’s an interesting back-and-forth going on at Thingiverse, a site founded by Makerbot to share 3D projects. Two designers have made two parts for the AR-15 rifle platform. The first part is a standard rifle magazine complete with spring but the second part is AR-15 lower receiver.http://techcrunch.com/2011/09/20/is-printing-a-gun-the-same-as-buying-a-gun/
Why are these parts important? Well, the magazine is just on the edge of Thinigverse’s implied (but not concrete) “no weapons” philosophy but the lower receiver is something else entirely. It is the only part of the AR-15 that you need a license to buy.