“For Sale” Data, is the “Oxygen” of real estate portals such as Zillow, Trulia, and REALTOR.com.
The deepest, richest, most accurate and timely comes from Multiple Listing Services (MLS).
MLSs are typically owned by an association of REALTORS, or a group of Associations of REALTORS. A group of Associations owning an MLS would probably be considered a Regional MLS.
An MLS is either a separate corporate entity of an owner Association(s), or a Committee of an Association.
The data in an MLS data base is acquired through the hard wok of real estate professionals, in most cases, REALTORS, who belong to an Association of REALTORS, and its MLS.
These associations are housed in unique building across North America…there are about 1300 Associations of REALTORS, and as of 2021 about 550 Multiple Listing Services.