Greetings Jim:
Here are your top three options, in order of priority:
1) Contact a professional consulting forester to assist you in the marketing of these black walnut trees. Professional foresters are experts in marketing timber for private landowners as they have no affiliation with the timber industry (i.e., they work for you, the landowner):
http://web.extension.illinois.edu/forestry/pdf/tfb-nres-202.pdf
2) Contact your local IL Dept. of Natural Resources district forester for a list of reputable, local timber buyers who procure from the greater central Illinois area where a lot of high-quality veneer black walnut grows:
http://web.extension.illinois.edu/forestry/pdf/tfb-nres-201.pdf
3) If the above two strategies fail, then by all means contact a licensed timber buyer directly http://dnr.state.il.us/law3/timber/09/TBlist.pdf
If you go this route, please know that you will be at a severe "selling" disadvantage because timber buyers are in the business of buying standing timber and logs -- landowners are not!