Trulia will attempt to take homes right in the general vicinity to compare for you where as a Realtor will do a comparison of like homes for you and take into account many things like Lakefront properties, special amenities that you may have, and much more.
Sometimes the websites like this are not 100% accurate so use the information with a grain of salt. Sometimes they can be accurate, but since it is a system and not a person, there are liable to be mistakes. They are good tools to get you started, but before making up your mind, meet with a Realtor for their opinion. It can save you tons of time and money.