Cal. Rev. & Tax. Code § 6241

For the purpose of the proper administration of this part and to prevent evasion of the use tax and the duty to collect the use tax, it shall be presumed that tangible personal property sold by any person for delivery in this State is sold for storage, use, or other consumption in this State until the contrary is established. The burden of proving the contrary 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.