California Code, Business and Professions Code - BPC § 21666

(a) Upon request, a vendor shall provide the purchaser a written receipt disclosing the vendor's name and address for any items purchased which has a selling price in excess of fifteen dollars ($15).

(b) No vendor shall offer or display at a swap meet any new or used personal property or merchandise of a kind which the swap meet operator has expressly prohibited. Every swap meet owner shall post or display in prominent places at the swap meet, or give written notice to every vendor prior to the commencement of a swap meet of the kinds of personal property or merchandise which may be not offered for sale or exchange.The swap meet owner or operator shall also post a sign at the main entrance of the swap meet which states that vendors are required to issue receipts in accordance with subdivision (a).