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).
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