I don't buy "excess of demand" as being fundamental, though, except in the most technical sense. Imagine a super niche event, where the organizers know there are exactly 200 fans on the entire planet, and they offer that many tickets on sale. It still makes sense for a random scalper to take a loan, buy all 200 tickets, and resell them with a markup. It would still make sense if the organizers offered 300 tickets, i.e. greater supply than demand in terms of heacount! A scalper could still come ahead buying all 300, selling 200 at a high enough markup, and feed the remaining 100 tickets to a cat.

I think scalping (arbitrage) can occur in any situation in which the supplier isn't charging the maximum the market can bear (perhaps that's what is meant by "excess of demand" and I'm missing some technical definitions here).