The New York Times ran an article last week about the increasing phenomenon of "pocket listings." Marc Santora's piece is here.
According to Santora, a growing number of sellers are choosing to not formally list their property for sale, opting instead to work through informal, word-of-mouth channels. Sellers' motivations vary, from a concern that the failure to quickly sell a listed-property could "tarnish" its reputation to an aversion to having to keep the property spic-and-span for showings.
The article alludes to (without fully examining) the attitudes of Realtors towards pocket listings and the potential legal and/or ethical ramifications of the practice, including the historical (one hopes) phenomenon of not listing properties as a means of racial discrimination.