While national portals like Zillow or Trulia can be helpful, their data is often outdated or even expired. 

36% of listings marked active on Zillow and Trulia were no longer for sale and only 4/5ths of the active homes on the market were displayed.

Zetimates (Zillow's home valuation tool) are also notoriously incorrect, so much so that recently Spencer Rascoff, Zillow's CEO, sold his house for $1.05 million, which Zillow then estimated at $1.75 million just a day later, thats off by over 40%!