Dave,
I found the quote below on
a site at the Chicago Kent College of Law. It says: "Interstate (auction) sales are those sales where goods are delivered to a buyer located in a different state from which the seller shipped the goods. Sellers are not required to apply sales taxes to transactions made with buyers in other states."
However, you should consult your own legal advisor to be certain. The firm that provides your auction software will probably also be able to advise you.
Robert