Cal. Rev. & Tax. Code § 6014

“Seller” includes every person engaged in the business of selling tangible personal property of a kind the gross receipts from the retail sale of which are required to be included in the measure of the sales tax.

For the purposes of this section, the phrase “tangible personal property of a kind the gross receipts from the retail sale of which are required to be included in the measure of the sales tax” includes all tangible personal property of a kind the gross receipts from the retail sale of which is, or would be, required to be included in the measure of the sales tax if sold at retail, whether or not the tangible personal property is ever sold at retail or is suitable for sale at retail.