faulty product and buyer want refund?

[area3222]

I sold a mobile hand set which was set as a brand new phone but opened once to check. Someone bid and won that item, but unfortunately product was faulty (according to buyer) and sent it back to me. I claim the warranty and manufacturer will let me know about this whether it is faulty or not. The buyer wants his money back as usual. But my question is, if I get the new hand set can I sent him back or legally can I force him to accept the handset, because he don’t want it any more. In my listing I put returned was not acceptable and he already open some accessories which is not in boxed condition. For those reason I can't sell it anymore as a brand new item though manufacturer only replace the handset not whole pack  . Please advise me about that matter.