Several options to consider...
1) Create an API and do a deal based on bulk data provider.
a. Based on monthly transaction queries.
b. Based on monthly transaction queries with ad/lead revenue share offset.
c. Bulk license data per state and utilize a smaller affiliated space company that has aggregated national but not a CoreLogic, First American, RealtyTrac etc.. You will probably find that most of the these smaller companies license from the big boys but could allow you the ability to query based on a revenue share. I know of a few India based companies that aggregate for larger companies and probably have the ability to license.
i. AVM Companies
2. Realty Bargains
3. Valuation Research, LLC.
4. Black Knight Financial Services
5. and the list goes on...
ii. Smaller Bulk Data Providers
1. Melissa Data
2. SMR Research Corp.
3. Realty Bargains - Data Company
iii. RE Space Companies
3. Realty Bargains
4. Home Finder
7. and the list goes on...
2) Not my first choice but you could develop a Scripting Engine utilizing property appraiser data. Look at publicrecords.onlinesearches.com
. You could leverage a company like this or a similar company and develop the last mile of the application. My guess is there are others doing it.