Kandace:
I'm going to assume your house and lot is located in a residential area -- yes? If yes, you will more than likely have to pay an arborist (i.e., tree service company) to remove these walnut trees as you're in a "tree removal" situation located in a residential area.
Homeowners should not expect timber buyers and loggers to be in the business of buying and removing yard trees -- even if your yard tree happens to be a black walnut.
In the extremely rare event that you may actually have several valuable sawtimber-quality black walnut trees located on a large, rural lot with absolutely no access issues and logistical issues, please contact a local timber buyer in your state of Missouri:
http://mdc.mo.gov/your-property/professional-forest-management/timber-management-and-harvest