Where to sell pretty valuable books?

I have a couple of books that according to the internet are worth a pretty nice chunk of change. First, how do I make sure the versions I have are actually the versions worth money and second, if they are where do I sell them to get some of the value? By my research I have nearly $400 worth of books here. They are in excellent condition.
In case it's important the books are The Hobbit and Harry Potter Chamber of Secrets American Version First Edition. If more identifying information is required I can give it.