Charlie Dresen gives a great explanation on how home sellers can, and do find a listing agents. As Charlie demonstrates in a sample online home search session a great way to choose a listing agent is to just pretend you are a buyer in the price range of your home and see which listing agent's listings stand out. Those are the folks you want marketing your home too.
The message here for real estate photographers is when you market to real estate agents don't forget to point out that when they use professional photography they will get more listings.
Oh, by the way Charlie is telling people to do this because he knows it works! This post on charlie's new listing a couple of weeks ago was an example of exactly this. The home sellers came to him to sell their home because they saw the marketing job he did with other listings he had.
Update March 2: Another way to use the technique that Charlie showed in this video is to look through the upper-end listings in your area to see what listing agent are using for photography. For those you see that you believe you can do a better job call them up, show them your work and make them believers in your quality work. Ethan Tweedie, on the big island of Hawaii just did exactly this. He showed me a $29M listing in his area that had just a few weak photos. I suggested he call the listing agent and pitch her on some quality photography. He contacted her it appears he has a new upper-end listing agent client. Pretty amazing story, this all just happened in a mater of a couple of hours! The moral of the story is you can find listing agents this way that with your help can become better listing agents!