This trade involves two assets, but it does not involve a sale. Also, this trade involves two parties, Alice and Bob. This leads me to believe that BitAssets is basically a loan for which the denominating asset (USD) is different from the lent asset (BitShares).
I think when you want to short BitUSD, there must be somebody, who buys them. This way BitUSD is "shorted into existence". If you want to close the short, you then buy the amount of BitUSD and cancel the loan using them. So there is a sale of the borrowed BitUSD involved here. Also 3 parties are involved: you, somebody who bought your borrowed asset, and someone else, from who you need to buy the asset in order to repay your loan.
This is how I understand the mechanics. If it's not like this, I would appreciate correction