National Trails System Willing Seller Act

National Trails System Willing Seller Act - Amends the National Trails System Act to: (1) prohibit the federal acquisition of land and interest in land outside the exterior boundaries of any federally administered area, except with the owner's consent, for the Oregon, Mormon Pioneer, Lewis and Clark, 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 the Oregon, Mormon Pioneer, Continental Divide, Lewis and Clark, and Nez Perce trails.

Authorizes appropriations to implement the provisions of the Act relating to trails designated by the Act as national scenic or historic trails. Limits funding for the acquisition of land or interests for, or for the development of, the Natchez Trace National Scenic Trail.

Instructs the administering agency for the Natchez Trace National Scenic Trail to encourage volunteer trail groups' participation in the development of such Trail.