Here is a good way to avoid all the extra B.S. that comes from sellers who do an inadequate packing job.
1) If suspect box arrives, take pics of the box BEFORE you open it, and at every step along the way.... and shoot close-ups of the physical damage, if there is any.
2) Now, here is the important part! Post your pics into a web-gallery (pbase, shutterfly, photobucket, whatever....) and include the link to your photos in your ebay messages. This is the only way ebay and/or paypal mediators can see the pics as well... More importantly, the seller/shipper will also knows that mediators can see those same images. You can even put that URL in the feedback if/when the time comes....
3) Don't ever lose your cool, stay professional and courteous. No matter how hard the seller/shipper obviously tries to push your buttons. This will only work in your favor in a dispute.
Having the images posted online in this manner quickly takes the wind out of anyone's sails who shipped out a shoddily packaged item and humbles them real fast.
I thought of this when I had won a Sansui amplifier on eBay a couple of years ago that showed up at my door completely smashed. After my first message to the seller (without pics) the response came back very much like yours. "that can't be, I saw the box and watched my friend package it, there were two layers of bubble wrap, what kind of scam are you trying to pull" yadda-yadda-yadda.
What the seller didn't acknowledge was that the bubble wrap was the really really big kind that look like little pillows and puncture real easy. His "friend" had wrapped the amp in that a couple of times and put it in a box without any tape or anything else. Granted, it probably did leave the shipper nice and tight. But, because they used the wrong kind of bubble wrap, by the time the parcel arrived to me all but one or two of those bubbles had completely burst and the amp was flopping around inside the box like a fish out of water. It was completely deformed in the process (not one of the better Sansui amps, mind you). After one look at the close up pics of the damage and the flattened bubble wrap, the seller immediately issued the refund. He knew I had him dead to rights. And I didn't have to pay to send the amp back, which is what eBay would have wanted me to do... The seller just told me to toss it as it was clearly beyond repair..
I know I'm stating the obvious now, but under no circumstances should you ever engage in emails outside of the ebay messaging system or resort to name calling. Keeping your cool will actually make some people even angrier as they are trying to get some kind of heated response from you. Believe it or not, some people really get off on this kind of drama and will egg you on even if they know you are in the right.
Overall, I have some 600+ ebay transactions under my belt and I've only had two or three that were like this. Ebay/paypal found me in the right each time. I've been quite lucky considering some of the stories I've read here and in other forums.