There is no such thing as a "minimum bid". Once you put up your auction, you agree to Listia's terms to give the item to the winner NO MATTER THE FINAL BID.
To set a minimum bid you need to click on "Advance Options" when listing an auction, then under "Start bid" type anything between 0-100. 100 credits is the highest minimum bid you can set and once an auction goes live you can't change it, unfortunately. Hope this helps! :)