The majority of hardwood lumber is sold by the board foot. Board feet is a measure of volume, whereas square feet is a measure of area. Therefore, in order to help you market your lumber, you will need to grade your lumber using the preferred methodologies of the National Hardwood Lumber Association:
http://www.ahec.org/hardwoods/pdfs/IllustratedGradingGuide.pdf
http://www.utextension.utk.edu/publications/pbfiles/PB1628.pdf
Depending on the grades, width, length, and thickness of your walnut lumber, might I suggest you contact several retail and wholesale outlets in your area (ex: woodworker clubs/shops, lumber outlets, newspaper adds, sawmills, etc.) to conduct a price comparison for the grades of lumber you have for sale.
I would also suggest that you surf the internet for walnut lumber prices (ex: www.woodfinder.com, www.ebay.com, etc.) as there are a plethora of companies and wood/lumber manufacturers who market their hardwood lumber inventories online.
Finally, feel free to contact your local state forestry agency and your state Tree Farm committee for local lumber/timber prices.
Best of luck!
