Not all real estate search engines are the same (doh!). In a fast moving market like ours today clients, especially sellers, want their listings to show up right away on all the various search engines This is especially true if there's been a significant change to the listings -- a price reduction, for example.
Here's a good example:
This agent had entered a price reduction into the MLS on 8/21/15.
The search results summary shown at the top show that Zillow and Redfin were still displaying the old price of $588,850. Clicking through to see the detail, Redfin actually shows the correct, reduced price but Zillow does not.
The likely reason for this discrepancy is the way these search engines get their data. Realtor.com gets feeds directly from the hundreds of MLS (multiple listing services) across the country. Zillow, Trulia and other aggregators also depend on direct feeds from real estate brokers as well as direct input from individual agents.
The bottom line for clients is: stay as close to the MLS for information as you can. Most MLSs in our area have a public version that anyone can search. For currently active listings (properties not already in contract), it's the most accurate source. You can usually set up (or have your agent set up) a search that will e-mail regarding new listings or change in status of a listing. The aggregators can do this too but, as shown above, it may take a while.