CA Rev & Tax Code Section 6421

(a)

If a purchaser certifies in writing to a seller that the property purchased will be used in a manner or for a purpose entitling the seller to regard the gross receipts from the sale as exempted by this chapter from the computation of the amount of the sales tax, and uses the property in some other manner or for some other purpose, the purchaser shall be liable for payment of sales tax as if he were a retailer making a retail sale of the property at the time of such use, and the cost of the property to him shall be deemed the gross receipts from such retail sale. The certificate shall relieve the seller from liability for the sales tax only if it is taken in good faith.

(b)

For purposes of this section, “use” is the same as defined in Section 6009 without the exclusion defined in Section 6009.1.