A reverse auction is a buying process. The buyer accepts the lowest offered price from a pool of sellers who will undercut each other. Compare this with a normal auction where you wish to sell something and accept the highest offer from a pool of buyers who will outbid each other.
In a day job, I worked on software that did normal and reverse auctions. We thought of changing the name reverse auction to tendering as this is a more straightforward description of what it is. See tender especially Webster's second and third definitions. This emphasises that it is not just a different kind of auction - there is a big difference between wanting to sell something and wanting to buy it.