There are theories about the true identity of the Ghost Salesman in Ocarina of Time. People, including myself, believe that the little boy in the graveyard, in the past, grows up to be the Ghost Salesman in the future. The boy spoke of a fascination of ghosts and all things scary. He even accepts the scary mask in the Mask Sidequest. They both carry a stick and swing it in a similar fashion.
Others believe it to be the Hyrulean Guard that is inside the same place in the past that wished there was more chaos in the world and was also fascinated with ghosts, but I think it's the little kid from the graveyard.