return item sent using a registered post tracking number

A return item was sent by registered post under another tracking number, not the one supplied by eBay. The seller denies they received it, although Australia Post confirms it was indeed signed for by the seller. How does eBay help in this circumstance when all efforts to find assistance through the eBay platform are met with the same case closed summary?

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I do not have a printer to run off the return label. Within the hour of receiving the item, it was returned by registered mail to the parcel  return addressee. Australia Post customer services have confirmed in writing that the item was indeed signed for by the seller. They have insisted they don't have it, based on the eBay return tracking number, but the registered tracking number mentioned has been received.

Contacting the department of Fair Trading is looking likely at this stage. The seller is in Australia.