You're giving a lot of credit to the seller. In the same way that HPD is sometimes wrong, the seller (private or dealer) is sometimes wrong. A lot of times (and I mean a lot of times), the firearms section does not have the right information.
FFLs/Dealers, the ones who handle the import of the firearm to the state, get the information (to include the serial number) wrong on the information they provide to HPD. The only person with the correct info is the person who has physical possession of the firearm. For the registration process, HPD puts the responsibility of providing the correct information on the registrant.
The online queue is similar to a physical queue. The physical registration is one-and-done because they are easy to fix at the registration window. The online queue can’t be addressed the same because it can’t be fixed quickly. Returning the request to the registrant for corrections assures that HPD will not be held liable later for registering a firearm incorrectly.
Of course the delays are error-caused. A good majority of the registration requests are the same requests that have been sent 1, 2, 3 times before. Imagine for a second if, when registering a firearm in-person, the registrant had to leave and return later with the correct info. Instead of being at the registration window once, they’d be there multiple times, waiting in line multiple times, filling out paper multiple times, increasing the wait time for registration; it’s the same concept.
So, you're telling me HPD doesn't know how to pick up the phone and immediately contact the registering party so they can process the submission?
I guess that's not surprising, since they can't properly communicate with healthcare providers in order to receive the critical mental health evaluations they need -- without the provider asking the patient to hand deliver it to the station.

It's not that I give HPD more or less credit than the seller. It's that almost all private sales are already registered with them -- they had all that info on the weapon and who owns it in their files. Handguns required permits -- which includes all the same info whether it's a private sale or FFL transfer. The FFL is directed by statute to keep and maintain proper records. If they are sending erroneous info to the police station when they make a sale, that's easy enough to fix. People straighten up when their license to operate a business is on the line -- something the ATF seems to be using now to shut down FFLs all over.
Again, I don't think those posting here made mistakes. If they did, they probably would have included the rejection/correction aspect in their comments. They aren't bringing in out-of-state firearms they already own, because those require in-person inspections. If everyone here is seeing a 2-4 month delay in getting their registration, there's only one answer: it's intentional.
HPD is telling people they can't USE their firearms until they get the registration in hand. I've registered dozens of times, and I've never been told that at the window. This is obviously being done because they know the registrations are taking too long. It's deductive reasoning, of course. No actual evidence or whistleblowers to prove it.
i think if it's not intentional, then they are just incompetent. I can get a driver's license mailed to me in 2 weeks or less. I can renew my vehicle registration online and receive it in the mail in a week or so. Every company and government agency that deals with money has procedures in place to expedite these kinds of transactions. The only reason it;s so complicated and error-prone is HPD doesn't care to make it better. The solutions exist. All they have to do is work smarter.
But, these are the same folks who waited until months passed to try and create new procedures schedule to take effect Jan 1. i refuse to accept the status quo as an excuse for denying a right.