California Code, Revenue and Taxation Code - RTC § 6041

(a) “Marketplace” means a physical or electronic place, including, but not limited to, a store, booth, internet website, catalog, television or radio broadcast, or a dedicated sales software application, where a marketplace seller sells or offers for sale tangible personal property for delivery in this state regardless of whether the tangible personal property, marketplace seller, or marketplace has a physical presence in this state.

(b) “Marketplace facilitator” means a person who contracts with marketplace sellers to facilitate for consideration, regardless of whether deducted as fees from the transaction, the sale of the marketplace seller's products through a marketplace operated by the person or a related person and who does both of the following:

(c) “Marketplace seller” means a person who has an agreement with a marketplace facilitator and makes retail sales of tangible personal property through a marketplace owned, operated, or controlled by a marketplace facilitator, even if that person would not have been required to hold a seller's permit or permits, or required to collect the taxes imposed pursuant to Chapter 2 (commencing with Section 6051) or Chapter 3 (commencing with Section 6201), or a fee administered pursuant to Part 30 (commencing with Section 55001), had the sale not been made through that marketplace.