The Multiple Listing Service (MLS) is the main way that agents input real estate properties onto the market for sale, and in many cases, for rent. There are different MLS around the country. Some states have only 1, while states like VA have more than 1. Sometimes multiple MLS will intersect to a high degree, with no 1 MLS having more than 50% of the properties in a given city/county (i.e. Williamsburg/James City County), while in other cities/counties, 1 MLS will cover >99.5% of the listings (i.e. Norfolk). Realtors and real estate firms need to pay for access to each MLS. Even when an agent has access to more than 1 MLS, they won't always put their listings in each MLS that they have access to due to the time-consuming nature of MLS entry and paperwork. From the MLS, especially when accompanied by paid programs that companies like Garrett Realty Partners use, the listing auto-populates to 100's of websites where buyers can search for listings.
Here is an MLS map of the US:
Image courtesy National Association of Realtors
MLS in SE Virginia
How to Find Out Which MLS are Used Most Outside of SE VA
Why Important for Sellers
Why Important for Buyers
Areas of Crossover MLS
How Working with an Agent w/ Access to a Higher # of MLS Can Help
If an Agent Doesn't Have MLS Access, What Can They Do?