How do you find a person who sells guns illegally?

It's not illegal...

If I want to sell a gun. I can sell it to a person I see walking down the street without even asking what their name is. If you've ever walked by a gunshow parking lot, that's something that actually happens.

They could have just gotten out of 40 years of prison, unless I explicitly know they can't own a gun, I literally and legally can't break any laws by selling them one.

Which is why lots of "private seller" don't ask names.

I don’t think that’s as easy as you think

If you don't want to wait on a gun show, lots of online websites exist to facilitate private sales.

I'm not saying it should be like this, I'm saying this is currently American law.

And there are a metric shit ton of loopholes, that no one seems interested in closing.