Cal. Rev. & Tax. Code § 6091

For the purpose of the proper administration of this part and to prevent evasion of the sales tax it shall be presumed that all gross receipts are subject to the tax until the contrary is established. The burden of proving that a sale of tangible personal property is not a sale at retail is upon the person who makes the sale unless he takes from the purchaser a certificate to the effect that the property is purchased for resale.