Jerry Ray, brother of James Earl Ray, made the announcement during a news conference at The Civil Rights Museum in Memphis, Tenn.
Ray has long maintained the bullet fired from his brother's rifle did not kill King, and he has spent seven years fighting in court to gain possession of the rifle to conduct tests. The State of Tennessee passed legislation in 1998 to retain ownership of the rifle and place it in the Civil Rights Museum.
The proceeds from the tape's sale will go to fund Ray's ongoing legal battle.
The Conspiracy Museum of Dallas, Texas, held a simultaneous news conference to announce its intention to purchase the rifle and conduct additional tests.
Memphis Judge Joe Brown said in July 1997 test data showed bullets fired from James Earl Ray's rifle did not match the bullet that killed King. Further tests were planned, but never conducted.