Greetings Dan:
Well, I'm not going to try to tell you how much your lumber is worth...too many variables; not to mention you're in California and I'm in Illinois. Try doing a little Web research to find out how much "green" black walnut is going for in your geographical area or else contact a local California extension forester.
Now, there are many ways to market local lumber: put an add in your local newspaper; contact a local lumber retail/wholesale store; contact a local woodworkers group; market the wood on Ebay (seriously, I'm not joking); market the lumber on other Web sites such as woodfinder.com; and ask your local California Extension Forestry office for additional ideas or leads.
Best of luck!