If I'm looking for sold comps in an area where I don't have MLS access, I use Trulia and Zillow, ONLY for the purpose of finding recent solds and listings. Then I take those comps and try to find the actual photos and details for each comp (from several sites) that still show the orig MLS data. I compare age, condition, location, type of sale from public record, etc. I NEVER use Zillow or Trulia estimates. They are VERY RARELY accurate! If it appears there's value in the property I'm looking at, then I'll let my Realtor dig her heels into all the data before making an offer. OK... you are probably wondering why I just don't let my Realtor do this from the get go. I've been doing this a long time, and I'm pretty darn good at valuations. Not that I don't trust the Realtor, but nobody looks out for your butt, as much as yourself.
I typically sell our properties for TOP $ (above closed comps sales) and don't have appraisal issues. I meet the appraiser at the property and furnish him with ALOT of data to support value. I know alot more about those SOLD comps that aren't really comparable, than he/she usually does. I use these same strategies when determining value on a property I'm interested in buying. If you don't have alot of experience in valuations, don't depend on zestimates.... make sure you have a darn good Realtor!