Most of the public is aware that the Multiple Listing Service is a private resource where Realtors list properties available for sale. Recently, the public has been able to access some of that information on such sites as Realtor.com, MSN HomeAdvisor, and others.
Once a property is sold and the transaction has closed, the selling price is posted to the listing in the Multiple Listing Service. Over time, it has become a huge database on past sales, containing much more information on individual homes than can be gleaned from the public record. This information is only available to real estate agents who are members of the local Multiple Listing Service.
Your agent will provide you with this data to help determine your offer price.