If no such transaction actually happened, then neither negative feedback from both sides is warranted. True that either of them wasted each other's time but is the feedback warranted for because no transaction actually happened? No. If they did the same to you as a way to get back for your feedback, then PM the staff to have this sorted and the feedback revoked for this reason. No negative feedback is justified unless the transaction has actually been done with less satisfaction from either buyer or seller.