From what I have been reading on these boards, yes, I would say that there seem to be a few scammers around who try to defraud people with altered labels, false tracking numbers, empty boxes etc-I wouldn't say it is common but it does seem to happen occasionally & usually with higher priced items.
I'd do exactly what k1000 sales said but I would also be ringing ebay and asking to speak to someone a bit higher up the totem pole. Be polite but firm and make sure they know you'll be going to the police about this as you have evidence and signed statements from Aust Post.
The problem here is ebay is in the unenviable position of being in the middle and what they are seeing is that the label printed had the 'correct' address and uploaded evidence it was delivered. The trouble with an 'altered' label is usually they have no proof of who altered it or when. However, in your case you are very lucky because you have a lot of outside evidence to support your case. By the sounds of it, very solid outside evidence from not only Aust Post (which I think is the strongest) but also from the recipient and the actual box with the altered label.
The buyer of course could claim the box did have the item and the recipient must have removed it. Is there a weight recorded on the box label, was it an item that would significantly change a weight reading? If it was in a box, then I am guessing it might have been. A very light box weight would further support your claim.
I know ebay staff are not trained investigators but I think when it comes to cases like this, they are going to have to put some staff time into researching all the details-even if these cases are sent to a special section. They can't just trot out a generic response. If they suspect fraud they should also report it to ACORN.
In this case if they feel they still have to refund the buyer, it should come out of their own funds.
But I don't think just uploading evidence to ebay is enough, you'll need to talk to them and insist they re-examine your case.
