[TJ]

Did you move to Washington from California and bring the tonner with you? or did you buy the tonner from a California resident with the California plates on it? If you moved and brought the tonner with you it should be easy. The Ca. plates were registered to you and all you need is the Ca. form that releases your interest in the plates. You can download that form from the Ca. DMV website. You can then sell the plates with the release form and the new owner can make a new application.
If you bought the tonner from a Ca. resident and that owner did not release the plates to you at the time of purchase then you have a different problem. You need to have the original owner of the plates sign the release of interest form in order for the plates to be used by someone else.