Selling a live black walnut tree
The value of the tree depends on the size and quality. Measuring a tree or log is called scaling and estimates the number of board feet of lumber that can be obtained from the tree or log. Tree scale is based on the DBH (diameter at breast height) which is 4 1/2 feet off the ground. So the diameter will need to be measured there. Then the merchantable height will need to be measured. This is the height up to the location where the tree branches out or about a 10 inch top. Once that information is known, the scale can be determined by using those measurements and comparing it to volume tables. These tables typically denote the height in number of 16 logs are in the tree. For example if the merchantable height is 32 feet, then it would contain 2 logs (each 16'). If the merchantable height is 24 feet, then it would contain 1.5 logs. The volume table will tell how many board feet is in the tree.
From there, you can compare it to stumpage prices to determine how much it may be worth. Penn State has a timber market report that reports stumpage prices that have been paid for various species but walnut is not one of the species listed due to the relatively small amount that is sold in Pennsylvania. I don't really have a good resource to tell you what walnut is selling for in Pennsylvania. The other option is to get a forester to come out and put a value to the tree for you.
I will also note that the quality of the tree has a large bearing on the value as well. How many knots there are, is there rot, etc. Here is a link to a publication that can assist you in scaling the tree: https://www.extension.purdue.edu/extmedia/FNR/FNR-191.pdf