Being an experienced eBay seller, this is what I would do.
The eBay listing and final value fee is gone, you can't get that back.
The postage money is gone, can't get that back either.
The PayPal fee will be refunded when you refund the buyer.
Do not send any refund just yet.
When you return an item to apple and they replace it, whether it's a refurb or a new item, they do not give you another full year of warranty. If this was the case people would return phones all the time and their warranties would always be valid. It just doesn't work this way with electronic items.
You probably had 5 months warranty remaining from the ORIGINAL purchase date, hence this transfers onto the new phone.
Did you keep a record of the IMEI number?
If so, contact Apple and ensure the phone is no longer blocked etc.
Did you list the phone as new? If so, this was your first issue.
The phone should be listed as a replacement, and therefore it is AS NEW. By saying it like this you do not let the buyer assume its new with full warranty.
When someone buys something from the net they have 14 days to return it, unless it was an auction and not a BIN.
If th eBuyer processes a PayPal dispute, she will win. No doubt about it. PayPal always side with the buyer. A major flaw with PayPal.
However, when a dispute is opened the funds will be held from your account, even if this means placing your account into a negative balance. The funds do not go straight to the buyer though.
The buyer will need to send you the item first with proof of delivery. Once she can prove the item was delivered to you, the refund will be processed.
You do not need to refund her postage costs, just the original amount she paid.
Make sure you check the item over, along with the IMEI number to ensure it hasn't been swapped etc before processing the refund.
You could always delay things even more for the buyer by claiming the phone has a mark on it to see what paypal do! Lol
If you paypal account goes into a negative balance you will need to clear it.
If you ignore it and don't use PayPal again, they have been known to get the bailiffs out, even for small amounts.
Unfortunately this is how it is with eBay. It's very one sided and the buyer always wins. They then have the cheek to allow the buyer to leave negative, and the seller can only leave positive or nothing at all!
I use GumTree for higher value items if I can so we can meet face to face. No fees. No fuss. eBay is a last resort with such items.