City of heroes also had this in their AH. It was awesome. You could put up bids for items and if someone put the item up for sale the person with the highest bid would get it as long as it at least met the price the seller was willing to sell for.

This was great for sellers because you instantly get the highest price for stuff that someone is willing to pay. Great for buyers because you can put up a low bid and just wait until someone is willing to part with it for that price.