Some vendors use additional enhanced property data, with information from appraisals or other sources. Only one vendor I spoke with, Solimar, will run an AVM without property characteristics on the subject. In many areas, particularly rural areas, no one compiles and sells the data. National AVM vendors compete on how many counties they can cover. A few AVMs use MLS data, but the user must subscribe to the MLS service.
Many MLSs are reluctant to resell their sales data as they see it as a profit center.
In many states, tax assessment information can be used to determine values. This is particularly useful for a local appraisal firm or lender, who is familiar with local assessment practices. If assessments are equalized in a neighborhood or city, all you have to do is apply a factor to the assessment. One of the major weaknesses in AVMs is a lack of a physical inspection of the properties. Most lenders I have spoken with using AVMs for home equity and seconds don't have anyone drive by the properties.