If your income dips down below $150,000 (or $100,000), then the same test is used as above. 

Next, you can advertise a home for sale yourself as a "test ad".

My tenant claimed there's mold in the unit (there was ~2 sq ft), I had an air quality test done in January, it passed - https://www.biggerpockets.com/...Fast forward to February, she claims there's still mold in the air.

They also have researchers who can look up missing data.

Some can be found by doing a records search and then piecing together the data for limited types of evictions.

GRM on a comp comes out low, well that is one data point. 

My goal is to find sales within 1/4 mile and 6 months, but I frequently don't have enough data without pushing out both radius and time. 

You'll have to run a test ad and see what the demand even is in this location and at what price point.

You can do a basic title search on a sheriff sale property like on any other property:  using a title company, attorney, or a visit to the country records office.This county's data may be online and searchable (sometimes for a fee), so you could try that, too.Best of luck!

If they see a wave of listings in a particular area and learn that there is going to be some major drainage work done two years from now that will significantly impact taxes, unless your going to meetings at city hall or live in the neighborhood, those are likely things you may miss.When you do begin to narrow down properties you like, they may be able to provide you with data that will reaffirm your own in regards to comps in the area, ARV, and the direction of price trends.