Zillow.com said its new, free service will let users load photos, descriptions of homes for sale and commentary on their neighborhood.
Homeowners can set a price they hope would close the sale quickly, and prospective buyers could contact them through an anonymous e-mail system. Real estate agents will be able to attach their contact information to home-sale offers they post and link to listings on separate Web sites, the company said.
Rich Barton, Zillow's chief executive, said the move would allow sellers to "plant virtual 'for sale' signs" on their properties.
The Seattle-based startup launched in February and uses county records and other data to compile its estimates on home value. In September, the company began letting homeowners add newer information about their properties to the public records that make up its vast database.
The services come as the Internet plays a greater role in buying and selling homes.
Home listings, once printed in books available only to real estate agents, are obtainable by anyone online, accompanied by increasingly sophisticated photographs and virtual tours.
Services such as Zillow are cropping up to help people judge house prices, survey neighborhoods and evaluate school districts, long before they ever snap the seat belt in their agents' cars.