What site can I use to find the worth of older books?

I have recently found several books datind back to the 1880's and I was just wondering if there are any sites that would give me the estimated value of their worth. Some of the books are "The Poetical Works Of Milton" copyright in 1881, Palmer's The Odyssey of Homer" copyright in 1880 and several others. If you know of any such sites could you please give me the urls?