He was not convicted for selling ammunition used in the Las Vegas shooting.
He [took a guilty plea] for being in the business of manufacturing ammunition for resale as a source of income without the business licensing.
The connection to Paddock is coincidental.....
It's also how he got caught. He'd been in the unlicensed business of manufacturing ammunition for sale for a long time. But he only came under ATF scrutiny when a box with his name and address and cartridge cases with his fingerprints were found at the crime scene. So as part of the investigation of the Las Vegas shooting, federal authorities had occasion to look at Haig and what he'd been up to.
This is an excellent example of how some crimes [Haig's] get found out. Haig was unlucky. Haig was caught and prosecuted because an investigation of another crime found evidence leading back to him, resulting in an investigation which uncovered his entirely different crime.
We often see folks say things like, "I know it's illegal, but no one will know." But folks who commit crimes can get discovered in all sorts of serendipitous ways.
Perhaps similar, but not absolutely identical. As we point out all the time in connection with legal issues, the details matter.
....My point is that for someone who casually dabbles in the gun world, it is unreasonable to expect him/her to know all the federal gun laws, licensing requirements, etc let alone all the gun laws for that his/her state and locality.....
And that is legitimately a problem. Having and using firearms is a heavily regulated activity, and it's easy to run afoul of the law.
In all the classes the instructor group I'm with put on, I point that out and provide a quick overview. I emphasize that I'm just scratching the surface, and I distribute a handout pointing the students to a bunch of on-line resources. We also give each student a copy of a thick book, written by a highly regarded RKBA lawyer, on California gun laws.
It's not going to be changing anytime soon, if at all.
On the other hand, prosecutors have a lot of discretion to decide when and how to prosecute. I suspect, but can't guarantee, that someone innocently, cluelessly, and with a pure heart who is caught committing a minor violation of one of the more technical gun laws might get off lightly. But there's no substitute for knowledge and avoidance.