Trulia uses many different databases for sales price information. Homeowners and real estate agents also contribute information to Trulia.
Trulia has a disclaimer about the data they use:
"Sales history and public records:
Prior sales are residential arms-length transactions. We receive regular updates from our data providers, but public record data may contain inaccuracies. Please contact your local assessor, tax, or recorder office to correct this information."
If you have found an error, you can report it to Trulia here: