Cal. Bus. & Prof. Code § 21506

Any person who sells a secondhand watch shall deliver to the vendee a written invoice setting forth the name and address of the vendor, the name and address of the vendee, the date of the sale, the name of the watch or its maker, and the serial numbers, if any, or other distinguishing numbers or identification marks on its case and movement. If the serial numbers, or other distinguishing numbers or identification marks have been erased, defaced, removed, altered or covered, this shall be set forth in the invoice.