Recall/Undo on Trades

I agree absolutely.  You should be able to cancel a posted order prior to it getting filled without being taxed for it.  In addition, I would suggest that the taxes not be collected until the sale is actually made, and then have that amount deducted from the proceeds.  It is possible that a player could try to post a sell order on the market because they literally have 0 gold and they need to raise funds.  If there is no gold in their coffers to pay the tax upon posting, they would be unable to sell goods to raise funds.  It's a perfect Catch-22 situation.  In the real world, a merchant would go broke if they had to pay the sales tax on every item that they stocked on their shelves before the item was sold.  Collecting taxes in advance will simply depress the market as sellers will find themselves unable to afford to advertise goods for sale, a situation which will lead to an untaxable black market.