Things can get broken no matter how well packed.
Ask you buyer to send a photo of the damage so that you can decided if you want the item back or not.
If you want it back the buyer sends it at their own expense by a tracked method of posting.
You then refund all the original payment of item and postage. It is up to you if you pay the return and if the item is so badly damaged then it is not worth having it back.
If the buyer opens a not as described dispute they will win so it is better to deal with it as amicably as possible.
Remember they can still leave feedback and dsr's and a lot of what they leave can be as to how you dealt with the problem