I believe you are referring to a Dutch Auction (http://en.wikipedia.org/wiki/Dutch_auction).

The fact that you don't know what the technical name is of the type of auction you are building would suggest that you probably haven't done enough research into the area.

You've not given enough info on what you are building, and perhaps this isn't the question you were asking, but you might want to research this some more before launching anything/taking anyone's money.