It does happen from time to time - usually for 
people who have found something else they want 
more. -_-;  You'll probably notice if you check 
what the person is bidding on that they chose 
something else after retracting their bid. I know 
of 2 people in particular who are really bad 
about this.  But yes, ebay does now have the 
ability to block people from bidding on your 
auctions and I have taken advantage of it with 
one person. :P  It is annoying, but unfortunately 
ebay doesn't do anything ... the person I 
mentioned has something like 90 retractions in 
the past 6 months -_-  At least they retracted 
the bid instead of just not paying though. Small 
consolation I know...