The Ecommerce Auction mod integrates into your shopping cart, some of the uses that it could be used for are:

1. You have old stock or end of season stock you want to get rid of.
2. To promote your store why not run a category of Products that are available at cost plus what the end user is willing to pay for them.

Check it out here http://demo.hcsdesignit.com/aspauction/placebid.asp