The ebay Resolution Centrer works very well (at least in my experience).
I was sent a broken Stanley No71 plane recently. It was a direct result of the seller's incompetent packaging. I immediately sent them a message explaining the situation and asking for their address so I could send the item back and telling them I expected a full refund. They did not reply. I gave them a week. I then raised the issue with the ebay |Resolution Centre. They got back to me immediately telling me to contact the seller to resolve the issue. I replied that I had already done that. They then immediately asked me to wait two days while they consulted. They then told me they had ruled in my favour and sent me a Parcelforce label to print out and put on the parcel (covering all postage costs) with up to 9 days to send the item back. I did so and received a full refund, including the postage I had initially paid. All up, it took a few days after raising the issue and they were very prompt and communicative. Thus, if you have any issues, I'd give the seller one try at resolving it. If the don't go straight to the Resolution Centre. If it's genuinely not your fault and you can clearly describe the issue, I think they will most likely rule in your favour and then give the seller a bollocking.