Ok when you are reported to eBay; you have to go through the process of the dispute. It can take some time. Also giving you some time to get the money back.
If the buyer makes a complaint to eBay then eBay will send you an alert stating you have to respond to the dispute. You have to respond!
You will have to have some proof. A receipt of purchase would be great proof that you could email eBay a copy. Hopefully you purchased the item recently before or during the auction itself.
Pictures are great too. Hopefully you have pictures of the item still in the box.
If you put text in your auction that you tested the phone to ensure working order in your auction he will automatically loose this dispute.
If you have the receipt and you can return it for an exchange where you purchased it. Then you can require the buyer to send the phone back for replacement then return it for an exchange with the dealer. The only thing you will loose out of the deal is return shipping and some of your time.
If the buyer files a PayPal dispute, you're in for some hard work. If you don't have the funds in your PayPal account then they could freeze your account altogether. Also stopping you from selling other items on eBay. You have to settle this dispute. You can't just hope it goes away cause it won't.
You really need to get the money for the refund. But he has to return the item, And this can buy you a week or two at most.
One cool trick that you can use in the future is if you ever do a listing do it as a picture. My auctions are pictures. That way you can edit the auction even if it's the last minute of the auction just by changing the info or text on the picture. Or even after the auction. But you didn't hear that from me.