No excuse for ignoring the buyer. A negative feedback shouldn't have to be left to instigate communication.
Edit:
This reminds me of one of my fruitcake stories. After a buyer placed an order, they left a negative feedback very quickly, "I DONT WANT THIS, CANCEL." If the buyer actually messaged me, I would have seen it immediately and canceled the order, but I didn't see the feedback until after I shipped, so I braced for an annoying return. Which is what I got! They shipped it back in gift wrapping paper (I shipped it in a box) and I got a great bonus. It went to an old apartment I hadn't lived at for years, because I never changed the shipping return address, which was independent from every other address I changed when I moved. So a little column A, buyer the fruitcake and a little column B, me the fruitcake.