A multiple-item (Dutch) auction is an auction where more than one of the same item is being sold at once. There are two kinds of Dutch auctions.
The most common Dutch auctions are actually a combination of two auction types: theyre multiple-item fixed price auctions (Dutch Buy it Now auctions to you and me). This just means that you can offer more than one of an item at a time for a fixed price.
This is very powerful if youre selling something small in large quantities. You can just say how many of the item you have, and the Buy it Now auction will stay there until its duration is up or all the items have been sold.
Buyers arent limited to only buying one item at a time, either: they can enter how many they want and then just click Buy it Now to get them. If youre selling small things loose, then this can be really great instead of selling them in packs of 50, you can sell 24 to one person and 95 to the next. It lets buyers save money by buying exactly what they need, and it lets you offer them the flexibility to have as many or few of an item as they want.
Dutch auctions can also be done by bidding, but...