No need to be concerned (though maybe for different reasons than you might hope for or expect)  - sellers can't leave negative feedback for buyers, at all. You can only leave a positive or nothing - if you leave a negative comment with a positive rating, it is reportable, the comment will be removed (but not the +1 to the buyer's positive tally), and you'll have a policy violation to boot. 

Best to simply go through the unpaid item process on all unpaid orders (unless agreed to cancel mutually), as that way no one can leave any feedback (or if they do, it can be removed).