Sellers can't leave negative (or neutral) feedback for buyers because too many sellers were using the threat of negative retaliation to discourage less than positive feedback from buyers no matter how warranted it was. And then if the threat wasn't enough, they would leave a scathing negative feedback for the buyer and seek a "mutual withdrawal." Since accurate and honest feedback for the seller is so much more important than for the buyer (all buyers have the ability to examine the seller's feedback profile before anything happens, while the seller is seldom able to do so and there are issues with taking action after the fact) eBay did away with negative and neutral feedback for buyers in 2008 and banned getting around that by leaving a negative comment on a positive rating.
On the other hand the Buyer Requirement that blocks those with 2 or more strikes in the past 12 months (or the more lenient settings available) acts automatically to block bids, offer, and BINs from serial deadbeats (IF their sellers used the proper unpaid item process) and so is far more effective than expecting to be able to react to feedback.
Also if you cancelled for non-payment after 4 days the buyer CANNOT give you negative feedback (or neutral or even positive). Neither party can leave feedback after the non-payment cancellation, and any feedback left before then is removable (not sure if that happens automatically or you have to contact Customer Service to get it taken off).
Of course there are other problems where your experience with a buyer can be "less than positive". For those be aware that the prohibition against leaving a negative comment applies only to feedback GIVEN. You can reply to any feedback s/he gives to you as appropriate. Sellers who know what they are doing check the feedback LEFT by the potential buyer, where your reply will be visible.
