When you sell a gun that's registered in Hawaii, HPD expects you to report it. So, if you sold the gun out-of-state, they'd expect a letter to that effect.
If I were to show up to register a gun that I claimed I bought on the mainland from Bob Smith, and Bob Smith had previously registered that gun in Hawaii, you best believe that HPD is going to pick up the phone and call Bob Smith. And, should Bob Smith still be a resident of Hawaii, they're going to want to know why, exactly, ol' Bob didn't tell them he had sold his gun.
Now probably no one goes to jail, or gets any form of legal action taken against them, but it will add to the hassle.