I believe there is a way you can set up your 
auction such that only pre-approved bidders can 
bid.  This requires people to actually read your 
description and contact you before the end of the 
auction so you can put them on your approved 
list.  It may seriously cut down on the number of 
bidders, especially last minute snipers.  I've 
never tried one before though, so I don't know 
the details.

Keys