Things shift around in any sort of packaging so by saying it was in the bottom when you packaged it, probably doesn't hold water..
Have you asked the buyer for pictures showing the damage? That would be a great help to see what happened....However, not everyone has a digital camera nor know how to use one or how to send pics thru ebays new pic sending option...
Good luck and you will propbably need to refund all the money, that buyer paid. I hope it wasn't alot of $..
Negative feedbacks come whenever, however, why ever, regardless of no matter what......They sting so all you can do is whatever the best you can do, will be...Sorry that this happened to you..
Happy New Year!