The number of homes listed for sale today is lower than it’s been in a decade. Home buyers are scouring Web sites for property listings and when they find a home they like, they often find themselves in an expensive, stressful bidding war. What they don’t know is that they’re not seeing as many as 15 to 20 percent of the homes for sale in their markets.

This is a critical time for buyers. Mortgage rates may never be so low again in our lifetimes. Though home prices have risen ten percent or so over the past year, they are far from the peak levels of 2007 and relatively affordable. Like mortgage rates, home prices have only one direction to go: up.

However, inventories or home for sale are down 40 percent or more from two years ago. Selections of homes, especially homes in the mid-range or entry-level price brackets. Even foreclosures are experiencing price increases.

You might do all that you can to find out about homes for sale in your price range. You can sign up to be notified by email when new listings come on stream. What you don’t know is that you aren’t looking at all the homes for sale. You are still going to miss one out of seven homes for sale.

That’s because almost all listing sites rely primarily on one source for their information: the nation’s 900 multiple listing services. Multiple listing services are databases of homes for sale. Most are owned and controlled by local Realtor organizations and only members of MLSs can list properties on them. The first MLSs were established 125 years ago and they have become more powerful with the emergence of real estate listings sites that have made it possible for consumers to see their listings. Despite the dominance of the MLS system, tens of thousands of homes are sold each year outside of it.

There are four kinds of homes for sale that don’t make it onto the MLS: pocket listings, for sale by owners, closely held transactions and expired listings.