The full MLS systems available to real estate agents distinguish between properties that are available for sale and those that are “under contract,” but the data making this distinction is not transmitted to the public MLS sites.

As a consequence, when you search the web versions of the MLS you can’t tell the difference between:
Active Properties: Properties that are actually available for you to buy.

Under Contract Properties: Properties where the owner has already accepted an offer from another buyer and is obligated to sell that property under the terms of the contract unless the buyer backs out for one reason or another.

The failure of these systems to make this distinction can render them very frustrating to use. After looking through all the properties in your price range, and maybe even driving by them to look at the neighborhood and the location, you call the agent and discover that someone else negotiated a contract on it two months ago.

Why isn’t this critical information on the public systems? Because the purpose of these systems is not just to market properties for sellers, but it is also a marketing tool for real estate agents and their services. For the seller, leaving the property on the system until closing continues exposing the property to new buyers in case the current buyer backs out of the contract for some reason. For the listing agent, this provides additional contact with buyers who call on the property. The agent can encourage the buyer to look at other similar properties that she has listed or can try to convince the buyer to work with them exclusively while searching for a new home.