I am not a seller. I think on most occasions it's good that sellers can't leave negative feedback however I also think in the case of non paying buyers (when provable) an exception should be made to this rule.

there are always going to be inconsistencies with any application of negs for buyers.

Take this situation, a seller has PayPal listed as a payment option, but then tells the buyer 'no PayPal'.  So the buyer doesn't pay as they want the protection that PayPal offers.  The buyer becomes a non-payer and the seller leaves them negative feedback.  Of course the buyer could appeal any strike and would rightly win that appeal.  What happens to the neg left by the seller because of provable non-payer?  Exceptions to rules create inconsistencies as the rule is no longer black and white.