#1: notify Ebay that they bailed, and they should offer you a free re-listing (should'va sold it to me for $200 - you'd be way ahead).
#2: Leave them scorching feedback.
#3: Go to the bid history on the item and contact the next highest bidder and offer it to them at their bid.
$100 money order? No note?
Yes, anywho is appropriate for calling them or writing them back to the return address.
Do you not have an auction that is due that amount??