If the buyer made a chargeback through their bank eBay is partly correct that they cannot override it. They could contest it, but this would require all the seller's evidence to be assembled and considered - something eBay doesn't even do when a buyer opens a case under eBay's money back guarantee!
In practice, as the chargeback is brought against eBay it's eBay's decision whether to contest it. Their policy seems to be the same as in their own MBG cases: generally to accept the buyer's word against the seller's. In reality, this is probably almost unavoidable because if eBay routinely investigated and challenged chargebacks for small amounts their costs would vastly exceed the admin fee they currently charge the seller.
The frustrating thing for sellers is that because a chargeback is between the bank and eBay, the seller isn't even a party to it and has no rights. We have all agreed in the user agreement to accept eBay's decisions, and to reimburse them for any payments returned to the buyer'.
eBay has become a very risky way to sell things. Their own money back guarantee is wide open to abuse by buyers, but even if this fails the banks and card issuers seem willing to reverse payments with little or no investigation.
