John,
You wrote "is it fair for the buyer to leave a negative feedback after the seller has left a positive." In my opinion YES. This is not you scratch my back I'll scratch yours. If the seller gets prompt payment that is his/her only concern. If the buyer gets the item in timely fashion and it is what they expected, that is their only concern. The gray area is when an item does not meet the buyers expectation. Then the buyer should contact the seller for resolution. If the seller is not willing to correct the transaction then why shouldn't they get negative feedback even though they left a positive??