Ticket scalping is essential for keeping ticket prices down. Yes it's a bad idea to buy a ticket from a guy on the street as there is a good chance he will swindle you in some way, however with online ticket scalpers like stubhub.com it has become very easy to get tickets cheap. For example, last season you could easily get tickets for the Chicago Cubs for around $6 a piece near the end of the season. Without these online ticket brokers I guarantee ticket prices would be much higher than they currently are offered from the professional sports teams.
If ticket scalpers cannot sell all of their tickets for top dollar, sports arenas and concert venues will soon acknowledge that their prices are too high. As a result, they won't be able to charge as much per seat. This follows the basic economical law of supply and demand. Of course, if they are able to get top dollar, prices will rise as a result.