"I was under the impression you can buy firearms legally without background checks at all in much of the USA"

I'm not sure if this is legitimate or if you're trolling. The regulations on background checks can vary depending on the state. Even in states where private sales are legal, you would still be prohibited.

Granted enforcing that prohibition comes down to people following the laws, and there's nothing physically forcing them to do so (whether requiring a background check or not), merely punishment after the fact.