In New York City, the largest agencies - as well as some of the small ones - want to choose exactly who they will share listings with.

Many brokers who question the time spent developing Lex see an Internet site as the place for real estate companies to share listings.

Nearly three decades ago, real estate agents across the nation began to make life easier for home buyers by sharing listings.

He added that Brooklyn sellers could already direct their brokers to share listings.

Manhattan brokers themselves took decades to start sharing listings on a marketwide basis.

Brokers, meanwhile, are reacting to the inventory crunch by increasingly asking that listings be considered exclusives rather than sharing listings with their peers.

Mr. Norton said that most listing brokers in urban neighborhoods were more territorial than suburban brokers and less likely to share listings.

Mr. Rubenstein said he had repeatedly and unsuccessfully lobbied Fandango's executives to enter into a so-called interchange agreement, in which the sites would essentially share listings.

Until then, it had only recommended that members share listings.

Small firms often say they are shut out of the lucrative co-op business because large firms do not share listings.