If you've played World of Warcraft, their npc vendors allows players to buy back the last dozen or so items sold.
Also, I have been using the sniper, which can work so fast that sellers don't even have five seconds to cancel their auction. Especially auctions accidentally priced too low. For example, I sniped 4 Rubeweed Tinctures for the price of 1 recently. (Items have been returned to seller.)