That's sad. Done right you can use card board as a separator. Try to work something out with the seller but first decide if you want to keep it. If you want it decide, ahead of time, what you'd like for compensation. You either want it or you don't.

Because you received it its within your right to leave negative feedback. I hate leaving negative feedback because 'things happen'. In this case, without question, I'd leave negative feedback. The seller, depending how big or small he is, may try to negotiate a 'no negative feedback' resolution which is completely up to you and within ebay policy. But be careful, 'feedback extortion' is against ebay policy and you could screw yourself. Don't say or hint anything like, "well, I'll leave positive feedback if you take $X off the price"