The problem with guns is that there are no laws governing them. There's a lot of wishful thinking, but that's about it.
Legal gun dealers have a hundred checks and balances they must follow and the vast, VAST majority of them do.
It's private sales. There are no rules. In nearly every state in the nation, there are no requirements at all on private sales. I can take my weapons carry license to about any gun store in the nation, buy anything I want without a background check, walk out the door and sell it all to anybody I want that even appears to be over 21 and it's all perfectly legal.
THAT is the problem. There should be a background check on every single sale, private or not. But that's why gun nuts and the NRA fight it: it would shut down their illegal sales, their income and their livelihoods...and that is the only thing on earth they care about.