Hi Garbrielle:
I received this exact question two to three times a week. Due to the great frequency of this question, I will email you a PDF document listing your options. However, please do not get your hopes up regarding "selling" your black walnut yard tree as there is over a 95% chance that you will actually have to pay someone to remove this tree!
Remember, this is a yard tree and yard tree removals are a paid service provided by certified arborists or licensed tree service companies!