Ebay don't specify any limits in their 'Payment dispute seller protection' policy but, understandably, the greater the amount the more reluctant they will be to provide seller protection when a chargeback is found against them as any reimbursement will be coming out of their own pocket.  Once the buyer's card issuer has made their decision it cannot be reversed.

Ebay provides protection against chargebacks in 2 ways:

1.  Where the seller has tracking proving delivery and the reason for the chargeback is either 'Item not recieved' or 'Payment not recognised/not authorised';

2.  Where an eBay Money Back Guarantee case of the same type has already been opened and resolved in the seller's favour.

Unfortunately for you, the reason given for the chargeback was 'buyer hasn't received a refund' where as the reason for the original eBay case was 'item not received' so you don't qualify for protection under either of the above scenarios.

The big problem that you have here is that because the chargeback is against eBay and not you there is no way of knowing what grounds the buyer has used for alleging that they 'haven't received a refund'.  As you are not a direct party to the chargeback you don't get to see the information the buyer's card issuer gives to eBay or the information eBay gives to the card issuer in order to challenge the chargeback.   While you may feel that eBay hasn't handled the chargeback correctly and simply passed financial liability onto you, it is actually in their interest to deal with chargebacks properly as any unchallenged or unsuccessfully challenged chargebacks will be counted against them and too many can lead to serious consequences. 

PayPal has no direct involvement here as this was a chargeback with the buyer's card issuer via PayPal rather than a PayPal dispute so they won't be able to provide you with any information. 

There is really nothing more you can do through eBay.  The only way you are going to get your money back is legal action against the buyer.   You have 2 key pieces of evidence in this regard:

1.  Tracking allegedly proving delivery;

2.  The messages you received from the buyer requesting a further discount or to return the item.

Do any of the messages from the buyer  actually acknowledge receipt of the item?  This will be quite crucial.