Guild Wars 2

Is it possible to cancel an order after it is placed, if the order has not been filled? If so, how?

press o, go to My Transactions, and select remove.

I tried to do that, but my offer did not show up in there. Sometime later I got the items at the price that I offered, so no big deal now, but in the future, it would be nice to know that if I change my mind, I could remove my offer.

You do it as TheUnNamedHero explained.

At the top of the TP window in the My Transactions tab there is a drop down box to let you view current sell orders or current buy orders (or historic data on past transactions). Select the appropriate option for whichever type of order you want to remove.