Maybe I'm wrong but I tend to think that if someone opened an account and bid on the same day it's a good sign. It means the were really interested in your item. So much so that they made an Ebay id!
That's the way I see it. Of course, many zero feedback buyers are non-paying but definitely not all. Honestly, the only feedback number I notice a decline in non-paying bidders is 500 & up. Before that number, it's basically fair game for non-payers :-/