1902 Harper's Encyclopaedias?

I have a complete set of Harper's encyclopaedia of United States history. The set was published in 1902. For their age they are in good, if not very good, condition. What is something like this worth and where else would I look into it? Where would one sell something like this? They were published by Harper and Brothers publishers, New York, 1902, London.

By jr from WI