Whenever a new marketing platform emerges, we tend to come up with the most straightforward way of using it without really knowing if it is the most effective.
In the case of Twitter, we usually assume that the best way of using it for marketing/selling is to create accounts and grow your followers, tweet "interesting things", etc. That may somewhat effective in rare situations, but the effort and ROI is huge and minuscule respectively. This works for Britney Spears, not real estate professionals.
A different way of looking at Twitter is as a vital piece of your overall Internet marketing strategy. Promoting yourself may be effective in certain situations, but when others are promoting you, the effect is much greater.
One example of how to get others to promote you is if you have a blog, integrate Twitter into your posts so that readers can tweet that they read your stuff and liked it, as well as provide a link. In this case they are doing your marketing for you, and their followers are more likely to visit your blog if it was recommended by one of their friends rather than you telling them directly. People listen to their friends, not to people who are trying to sell them something, even if you're good at masking it.
This type of strategy can be used on any type of internet presence besides blogs. It can be applied to your website, your online listings, online classified ads, or anything else that is web-related.
The goal is to let others promote you, as that is where the power of social media lies.
Also, the key is to make your various online promotional efforts leverage off of each other (ex: blog -> twitter) to achieve maximum benefit.
Agree/disagree? I'd like to vet my theory.