I totally disagree with that idea.
Players should be encouraged to the sell their items for their real value. If there would be no tax, I could keep an item forever or even buy every single item of it's type and create a monopoly. Then, when I would list these items in auction for the same price for months, there would be no loss and at some point players would be forced to buy my items.
You say, that people would put more items in auction and benefit the economy, but in reality the prices would stay the same forever, since there is no tax/punishment for unsuccessful sales.