I realise that this is a topic that you have strong feelings on and while I appreciate the effort you have made to contact the OFT in the hope of obtaining a 'concrete' answer, I'm slightly puzzled why you chose to direct your question to them for a couple of reasons.
1. The OFT deals only with purchases from a registered business and therefore a significant number of eBay sales fall outside of the OFT's reach.
2. The OFT cannot offer legal advice, indeed the vast majority of their staff have no legal training whatsoever and only specialised units have any powers of enforcement.
A lot of people aren't going to like the answer but in reality the answer is about as straight-forward as anything ever gets in law.
First things first, an eBay sale constitutes a contract. This is not in dispute (Smythe v Thomas). Every contract contains a number of terms that must be met and provided that these terms are lawful and not considered unconscionable (unfair to a particular party), once each of these terms have been met then neither party has any recourse against the other provided that neither party has acted negligently or fraudulently.
Title passes when agreed upon, in the absence of a specific term then it is implied that title passes once payment has been made for the item in full. In effect, once payment has been made the seller becomes bailee until the items are sent as per the terms of the contract, he/she remains responsible for the item until the terms of the contract have been met despite have no financial interest in the item.
Let's assume that we have sold an item that we intend to send by standard (non registered/insured) post as per our eBay ad. Upon winning an auction or clicking buy it now, the buyer has accepted your terms and the contract has been formed, the buyer has agreed to standard carriage unless agreed otherwise at a point prior to the sale. While obviously the seller has a legal obligation to take reasonable steps to ensure the item isn't damage prior to postage, once the item has been posted as per the terms contained with introduction, the sellers obligations cease and the contract is considered performed.
Whether or not the item is ultimately delivered is of no legal consequence to the seller by virtue of the various Sale of Goods Acts (and similar) throughout the various states. In QLD the law states -
Unless otherwise agreed, the goods remain at the seller’s risk until the property therein is transferred to the buyer, but when he property therein is transferred to the buyer, the goods are at the buyer’s risk whether delivery has been made or not.
As title passes to the buyer upon payment being made in full, excluding the sellers obligations as a quasi-bailee there is no obligation on the part of the seller to ensure that delivery is ultimately made. Once it has been posted then that is it, end of chapter. Of course it all changes if in your ad you state 'I guarantee delivery' but I don't think I've ever seen an ad with this term.
Paypal clearly understands the law, hence why you need only prove postage and not delivery to be eligible for seller protection. If delivery was required under Australian law, I guarantee Paypal would require proof of delivery every time a not received dispute is made.
This now brings us to the terms and conditions you agreed to when placing an eBay ad and accepting a Paypal payment. It is extremely important to note (and this is something that I think a lot of people struggle to comprehend) is that eBay and Paypal conditions do not supplant statutory and common law, that is where there are inconsistencies between Paypal's conditions and Australian law, the law wins out every time.
What this means is that although Paypal require, at a minimum, a postal receipt with the buyers postcode or a tracking number of some description, it does not mean that a court or tribunal would uphold these terms.
Assuming Paypal have found in the buyers favour, you would have to lodge a claim with the appropriate body in your state and being the complainant, the burden of proof would rest with you. In civil matters, this standard is considerably less than the criminal 'beyond a reasonable doubt'. The civil standard is 'on the balance of probabilities' meaning that you need only demonstrate that you 'probably' posted them item and your claim will be successful.
My knowledge is restricted to QLD but I'm aware of QCAT accepting photographs of parcels and even feedback indicating that there have been few or no aggrieved buyers as evidence that the items have indeed probably been posted. Paypal aren't silly though (name one multi-billion dollar company that is!), where they are of the belief that they are unlikely to successfully defend a claim then they will settle. If you have 4 or 5 figure feedback at 100% or close enough to it, this is likely to be enough on its own for your claim against Paypal to be successful.
To summarise, there is no obligation on the seller to prove delivery. The seller need only prove postage and there are substantial differences between Paypal's and QCAT's (and presumably interstate tribunals and courts) definition of proof should the dispute proceed that far.
