Any citizen that wants to make sure they are not selling to a prohibited person has the option to walk into an FFL and have the store run a background check. Police departments/government do not have to be a part of the process at all, as it should be.
A simple drivers license check would answer most of the questions: legal age to purchase, legal resident of the state, real name & photo.
I always ask the buyer to send me a pic of their license so I can fill in the bill of sale ahead of meeting.
Once you have their name, you could run a query on the state's judicial database for any cases. If they were convicted in Hawaii, that should be in the database. Won't include convictions in other states, though.
And if they hand you a state ID instead of a license, you might wonder why they don't have one. Too many DUIs could lose them their driving privileges, which could also lose them their firearm rights.
So, if these or any other red flags are preset, you might ask if they are comfortable with you running a background check first. Chances are, they will say forget it if they know it'll be denied. At least, they should say that.
Yet, felons still go to FFLs and attempt to get guns just to see if they can slip past the check -- and they almost never suffer consequences for lying on the 4473. So, your buyer might do that, too, and waste your time and money. Maybe asking for a deposit up front to cover the UBC cost would protect you from losing that?
Trying to hash out all these details of how a UBC process would work, what your risks and liabilities are as a seller, and whether or not the checks even work at all have never been brought up because the long pole in the tent is still the fact that the scheme fails without knowing who has what guns -- a registry. if you can't enforce the law without a registry, there's no incentive to follow such an unenforceable statute.
People speed believing they won't get caught, even though LE is on the roads everyday trying to enforce the speed limits. Yet, the odds of getting caught are slim. If you think people are obeying the law, you haven't been driving lately. If you drive the speed limit, you're being passed and impeding the flow of traffic.