It is fairly easy, as the New York Times reports, to buy the parts needed to assemble your own weapon and bypass the reporting laws that a retailer would have to comply with to sell a gun.
The Times assembled a weapon that had no serial number and would be untraceable. Then it tested it out.
No, it is not easy to build one. But for a felon, for example, these do-it-yourself kit guns make it easy for somebody who is not supposed to have a weapon to get one that cannot be traced.