Unfortunately in this age of technology information about everything is available. There are several classes of protected information however so speak with Trulia directly. They are a stellar organization and will assist you as best they can.
Douglas Montgomery, Broker/Owner
HomeNY Corp. Douglas@HomeNY.com
In most areas the sale price of property is public record and recorded at the courthouse.
I don't know if some of these sites are nice about it and take the info off by request, but I'd bet that this isn't the only site you can find it on.