First off, let me give you my smarta$$ answer.... I hope they are veneer grade and he is bringing them to my mill.
As I said before, there is a lot of risk in buying logs. Back when I was still living in Williamsburg VA, I had a customer bring a Black Walnut log over from West Point for me to saw up for him. I jumped up on the trailer and looked the log over before unloading it to the mill. After a quick inspection ( mostly looking for metal ), I thought he had a nice log and was looking forward to seeing what kind of color and grain the boards would have. Imagine our surprise when cutting off one of the slabs we discovered the log was hollow and filled with rot mud. The rot started about 6" from one end and went to about a foot from the other end. A much closer inspection of the slabs found 2 small holes from dead branches about the size of my little finger. We should have gotten about 120 bf of nice lumber from that log, instead we got about 20 bf of low grade wood.
When I first saw the rot, I offered to kick the log off the mill and not charge him anything for my time. He wanted to keep sawing just to try to get something out of his time and travel. He even paid the full price for my saw time. As that was our first meeting, I never expected to see him again. Turned out to be a good customer and brought over a number of logs for me to saw. Ask me about sawing a Cedar log for him if you want a real sad story.
Back to the question about a load of Black Walnut.
The OP needs to do some research to learn the different values of different types of logs and how to grade them. He also needs to visit a few mills in the area to learn which mills are looking for what kinds of logs.
Larger mills today seem to focus on a smaller ( and more selective ) segment of the business. Some mills want only one or two kinds of wood. Some only want logs in a certain size range, such as 10 - 16 inches DIB. Because of this specialization, some mills may have no real desire to saw nice large Black Walnut logs. (However, they may buy them and resell to another mill that saws hardwood for grade. )
A small time logger or a tree service that has a wide range of log size and types, may get a better return by selling their logs to a forwarding yard or a small mill operation. Another option would be to have a portable mill come in and turn the logs into lumber for you. You could then sell the lumber green or build yourself a solar kiln and dry it for an even better return on your investment. ( I still drag my mill back to Williamsburg - 8 hrs each way- about twice a year for customers I still have there.)
Many of the state universities in tree country, work with their state extension office and publish timber prices for the area. Try to google "timber prices" or "log prices" for more info.
Rick