I think it depends on the situation. I know that 
I have sometimes bid on many cels at once on eBay 
assuming that I wouldn't win them all or that 
some would never reach my maximum bid. But once 
or twice I've been wrong, been flooded with 
winning emails, and been too broke to afford them 
all. Although I did pay all of these people, I 
would have rather backed out if I could then 
practically go negative in the bank. If someone 
is in a financial crisis of any sort 
(unexpectedly) then sometimes that is excuseable. 
I would rather know that I am giving someone a 
chance to get back on their feet than make them 
buy my cels and be worse off for it.

But in most cases, I think banning is a fine 
thing to do. You are really just protecting 
yourself against non-paying people in the future. 
Cel dealers and people who sell on eBay usually 
only do so when they need them money. If someone 
wins or orders a cel but doesn't pay, you may 
have lost out on someone who would have paid for 
it. So sure, ban away. ^_^ Just make sure you do 
it nicely.