For purposes of determining the total combined sales of tangible personal property for delivery in this state pursuant to Section 6203:
(a) A marketplace facilitator shall include all sales of tangible personal property for delivery in this state, including sales made on its own behalf and by all related persons and sales facilitated on behalf of marketplace sellers.
(b) A marketplace seller shall include all sales of tangible personal property for delivery in this state, including sales made on its own behalf and sales facilitated through any marketplace facilitator’s marketplace.