What Ebay states is that user and seller should 
contact each other in 3 days- on my auctions I 
have started putting that people must contact me 
within 5 days and payment must arrive in 2 weeks 
(of course I'm usually willing to work something 
out if the person emails me).  If the person 
doesn't contact me by then, I usually offer the 
item to the next highest bidder.  This happened 
to me once where the person did not respond to my 
3 emails untill 3 weeks after the auction for 
some anime tapes- I had already sold the tapes to 
someone else and so I sent the person back a 
money order for the amount they sent me- after 
that I started putting the line in my auctions 
warning people to contact me within 5 days.
Supposedly ebay will pay you for the item if a 
person doesn't pay- I've never tried persuing 
that.
also- I have had a 0 feedback person buy a cel 
from me, and it was fine.  As far as I know he 
now has 1 positive feedback. ^_^ worry much more 
about negative feedback than none...