Online resources like Trulia or Zillow and others are very useful as a quick guide to information. But the question is where do they get the information? They get it from the Multiple Listing Services (MLS) of real estate agents across the country. They can only keep and so much data and do so to provide general information for consumers. They are a great guide and introduction.
But when you want a more thorough look into the data, the local MLS maintained by real estate professionals will have more information going back much further. Information is everywhere now but what is most valuable is the ability to select, interpret and analyze the vast amounts of information out there. This is especially true in real estate.
A real estate professional will have access to the kinds of data you want and can also help you refine what you are trying to find out. Real estate agents with experience have been through more than a couple of market cycles and can put the data in context.
I have found that being an ongoing resource for my clients even when they are not in the market is as important a part of my work as heloping with buying or selling. Any agent can pull out the data you want, but I recommend asking someone you can trust and will be there over time when you need it and can helop you use the data to its best advantage.