The officer at the door of the registration and records department told me that it is possible to register a rifle you obtain from a private seller/resident in Hawaii using either (a) the bill of sale and serial number of the rifle, or (b) the bill of sale and the physical rifle. He couched his answer with a recommendation that (b) is the preferred method, in case of issues.
Without getting into a long discussion of what might be issues, here are some possible hitches. Four rifles were stolen from J Hara Store a few weeks ago. What is to stop a thief from presenting a phony driver's license and a forged certificate of registration for a rifle? Second example of malfeasance is an owner modding a rifle so it fires automatically, possesses a bump stock, or is equipped with a suppressor. Third problem could be the seller forgetting to ever register the rifle in the state. Unless you are good friends with the seller, and trust him explicitly, you have several alternative options. Broker the deal through a licensed gunsmith, or bring the seller with you to the HPD office. Today was a very busy day at HPD. However, one buyer dragged his seller into the gun inspection room, where the officers asked a bunch of questions, allowing the buyer to leave with a certificate of registration.