Greetings Edward:
Seeing that I receive this question quite frequently, my common recommendation is to contact a local consulting forester or a local state agency forester to assist you in determining the marketing potential and value of these black walnut trees.
It is too difficult to assess marketability and merchantability of individual trees via this forum, thus an on-site inspection and evaluation by a trained forester is strongly recommended.
Please conduct a Web search to locate a professional forester in your your respective state of residence :-)
Best of luck to you and hopefully, if you're lucky and savvy in your marketing, you can find a buyer for these black walnut yard trees assuming they contain merchantable material!