I've read that you can't buy handguns directly from the seller in any state if you're not a resident there. The seller would have to ship it to a Hawaii FFL holder FROM an FFL in the seller's state. Then you will be able to pick it up once you return to Hawaii and jump through all the permit hoops: permit to acquire, 14 day wait, etc. Also, a Hawaii FFL won't accept a package from a non-FFL sender. The Hawaii FFL holds the handgun until you produce the permit to acquire.
Long guns are different. As long as you are old enough, per that state's law, then you can buy and possess without being a resident there. Most mainland states don't register long guns at all.
Then, you can use a TSA-approved container to fly them home with you -- maybe paying the airline an additional "checked bag" fee as well. That's when the Hawaii 72-hour rule for out-of-state-purchased guns kicks in. If you don't have your long gun permit, the HPD firearms unit will be more than happy to look after your new long gun(s) for 2 weeks.
I've not done this, but that's my understanding based on reading. I'm sure someone will correct me if I'm wrong.

Hope it helps!
