It seems as though the important part here wasn't so much that he sold them himself as it was that he added the following clause:

"You’ll see that if you try to sell the ticket anywhere for anything above the original price, we have the right to cancel your ticket (and refund your money). This is something I intend to enforce."

This is something that any middleman could do, too... but the incentives simply aren't there for them to prevent scalping. Especially as Ticketmaster has its own resale site, TicketExchange.