I have an eBay store so I deal with NPB all the 
time (not cel related though).  I'm totally used 
to it now and just sort of see it as a fact of 
life on eBay. ~_~  I don't get worked up over it 
anymore, I just process the reports when the 
deadlines are up and thats that.  I guess its 
just a matter of getting used to it.. Thats 
probably not much help though :\  Theres really 
nothing you can do to prevent it, 
unfortunately.  Ebay is implementing (or maybe 
it is already out?  I'm not sure) an option 
where you can force people who use Buy-It-Now to 
pay immediately via paypal - they dont register 
as the winner until they complete the payment.  
Thats one way to stop some NPB but it would also 
severely limit potential bidders.

I don't like to deal in private transactions 
because I find that people will back out even 
more in those situations because there are no 
consequences.