Determining the amount of inventory that should be written off because of obsolescence is a difficult and challenging audit task because (1) the client will usually state that most of the goods are still salable at current selling prices and (2) net realizable value is only an estimate (in other words, there is no specific, correct price at which inventory should be valued). Because of this, the auditor usually gathers corroborating evidence to provide evidence on valuation and, relatedly, obsolescence. Identify at least five sources of such corroborating evidence.