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Before answering this question, lets think why do we need a transaction ID at all?
Because we need to differentiate various transactions that are possible on an item, and find a particular sale(Order) on an item. This makes sense when the item quantity >1 .
Consider the case of an auction item. Where the quantity of the item is always 1. There is no possibility of transactions > 1. if the sale is an Auction then the ItemID is all that is required to uniquely identify the sale.So, TransactionID is not really required in case of Auctions(even if they have a BuyItNow button, still the quantity is 1).