Ticket Brokers work as a business - they work on supply and demand. They do this the understand that they will follow a strict code of ethics. Ultimately it is how they go about there business that differs from the practice of scalping tickets. Using a reputable broker ensures that should the tickets you purchase be a fraud you would have little recourse if you bought them from someone on the street. Where as, a web site like stubhub will give you an avenue for getting a refund.
Ticket Brokers are supposed to act in your best interest. They should give you as many options as they can - where you can find tickets at the best prices if they don't have tickets that fit your needs. They should also provide you with information about who are the best resources and most reputable.