Ariz. Rev. Stat. § 42-5011
For the purpose of computing the tax levied by this article, conditional sales shall be treated as credit sales and the tax shall be paid only upon the amounts received under such conditional sales contracts, but if the seller transfers his interest in such contract to a third person, he shall pay upon the full sale price of the commodity unless a record is kept of payments thereafter made on the contract in such manner that the department may at all times ascertain from the records of the seller the amount paid thereon by the purchaser. If at any time the department cannot so ascertain the amount paid thereon, the tax shall become due and payable as to any amounts not shown to be paid by the records of the seller or to the satisfaction of the department.
A.R.S. § 42-5011