Hi there !
I have faced some buyer's like that in your case. And for every seller, these kinds of fraud buyers are always bad for the business. I can understand how you might be feeling right now being cheated in that case. Here i have few suggestions for you, please go through them.
1.If you haven't, then communicate with the buyer through the eBay's "Messaging System". And try to get a proof that he is blackmailing you for the feedback. Because this is a feedback extortion case and under the seller protection program such buyer's are strictly banned from eBay.
2.If the buyer has raised a guarantee claim against you, then send one mail to "insellerverificatin@ebay.com" explaining all the details about this case. And request them to handle this case on priority basis because the buyer blackmailing in it.
3.Talk to the eBay's customer executive after sending the mail and ask them to transfer the call to the high level executives. Because most of them will just ask you the problem and give you the routine reply. Explain them the whole story related with the claim id that you have received.
4.Most important thing, keep on talking to the buyer and ask him to return the phone if he is not happy with it. Because mutual understanding is the best way of communication in any online business. I deal with such buyer in my own way but i can't reveal the same in public 😉
5.Don't worry a lot. Because even if the claim is resolved in the buyer's favour then also you will get you item back. The only bad thing for you in this case will be the negative feedback received by the buyer which is not a very big thing if you ask me. Because you are doing your job really well and you can maintain yourself to be a high rated seller in future even without 100% positive feedback.
Good Luck !