Your best bet, due to the simple fact that these are "pasture" trees, is to contact a licensed timber buyer directly.
IL Timber Buyers: http://dnr.illinois.gov/timberbuyers/
If you had more trees or if the trees were located in a non-pasture area, you would probably be better served by contacting a professional forester to assist you in marketing said black walnut trees.
IL Consulting Foresters:
Best of luck!