National Trails System Willing Seller Act

National Trails System Willing Seller Act - Amends the National Trails System Act (the Act) to: (1) prohibit the federal acquisition of lands outside the exterior boundaries of any federally administered area, except with the owner's consent, for the Oregon, Mormon Pioneer, Lewis and Clark, Iditarod, and Nez Perce National Historic Trails and the Continental Divide, North Country, Ice Age, and Potomac Heritage National Scenic Trails; and (2) provide that federal authority to acquire fee title shall be limited to an average of not more than one-quarter mile on either side of such national historic trails and the Continental Divide National Scenic Trail.

Eliminates requirements under the Act regarding: (1) the prohibition on the acquisition of lands and interests in the Potomac Heritage National Scenic Trail and the Nez Perce National Historic Trail outside of the exterior boundaries of federally administered areas for such trails; and (2) the limitations on the designation of lands outside of such areas as segments of those trails.

Revises Act provisions authorizing appropriations for the establishment and designation of national scenic and national historic trails.

Limits funding for the acquisition of land or interests in land for, and for the development of, the Natchez Trace National Scenic Trail. Instructs the administering agency for the Trail to encourage volunteer trail groups' participation in the development of the Trail.