Thank you foxette and digital ghost for clearing that up for me. This is the first time this has happened to me and I was not totally sure how to deal with this. I just don't like how I have to prove that I have done the right thing when the situation is an unauthorised payment, if they where claiming I had not sent the item, then I would understand why I would have to prove that.

It's a very unfortunate situation - what you are essentially dealing with is theft. That is, at some point, either when the item was paid for, or when the chargeback was initiated, someone else has done the wrong thing and sadly, if you can't fight it, you may have to pay for someone else's fraud.

There are more experienced eBayers around that can offer better advice on how to fight it without proof of shipping, and/or how to deal with the buyer, so I will leave that to them (hopefully they'll see and respond to this thread). I hope it works out.