Here is the best answer I can give you… it will be somewhat vague.
Every county in the country has a local association of realtors and each association has a multiple listing service. In San Diego SDAR is our local association and the MLS is run by a seperate company called Sandicor.
The MLS makes money by providing the MLS database to 3rd party providers. Generally this is known as IDX. These 3rd party providers such as Realtor.com, Zip, etc purchase the database. Some of the MLS information is stripped prior to shipping it out. This includes confidential remarks, buyers side commission etc… So in general yes these 3rd party providers do have everything on the MLS. However it is usually a day or two old.
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