Thats a good question. I guess each state can have its own laws, but my best understanding is that it is not necessarily illegal to scalp tickets, just that you may not be able to do it within a certain distance of the event, to avoid undercutting the event ticket office. Also, some concern about the selling of counterfeit tickets, I guess. But beyond that, I dont think reselling a ticket is illegal, though if you buy a ticket that is ‘non-transferable’ then selling it to someone else may be another legal issue.
It all depends on what state. But pretty much all of them are on the same page.
1931 law in Michigan (Michigan Compiled Laws § 750.465) makes it illegal to resell a ticket above its face value price without the written consent of the event operator and venue operator. ..