Cal. Rev. & Tax. Code § 4530

Any tax certificate may be transferred at any time before it is canceled by endorsement by the tax collector, if the tax certificate is held in physical form, or by electronic transfer, if that tax certificate is held on book-entry form. The tax collector shall modify the tax certificate to indicate the new holder. The official endorsement of a tax certificate by the tax collector with the date and its entry on the tax certificate record, together with a notation showing the new holder, shall be sufficient evidence of the assignment of the tax certificate.