What don't you understand? Who would go through the process and spend the money if they weren't dealers (or the other categories)?
Answer: Those who intend to use the license only to get guns cheap. Those who do so are dishonest.
Gentlemen, perhaps I can help here. Recall,as hard as it might be to believe in today's world,prior to 1968, EVERYONE bought and sold guns.
Firearms and ammunition were a staple of Mom and Pop Hardware stores, Sears Catalog was famously the source for many first rifles and shotguns for 12 year old teenagers, every High School had a Rifle Team and it was not uncommon to see 12 year olds with uncased .22 single shot rifles strapped to their bicycles or riding the Subway.
When the 1968 Gun Control Act was pitched and sold all the usual reassuring voices popped up with the now familar argument: IT IS NO BIG DEAL, just send in your ten bucks, get the silly piece of paper and return to business as usual just as you did in the "before times".
We now, of course, realize it was a VERY BIG DEAL and has led to significant harm to untold numbers of "average citizens".
Recall also in the sixties and well into the seventies the minimum wage was around $1.25. A ten dollar license fee was a significant investment for people who previously had to go to no expense to buy the rifle or pistol of their choice.
In short, the debate should be about whether the ATF should exist, not whether the rules are fair or just.
Perhaps this will help you understand the perspective from yesteryear.
Best regards,