I don't appraise timber trees or yard trees via this forum as it is impossible.
Are most black walnut yard trees valuable? No, most walnut yard trees are not valuable. Is it possible to sell a black walnut yard tree? Yes, there are instances where homeowners were paid for their black walnut yard trees; however, such examples are generally considered the rare exception rather than the rule.
Please contact a local logger if you TRULY want to know how much someone would pay you for your black walnut yard tree. Use the Internet to contact your state forestry agency to obtain a list of local loggers.
Best of luck!