Can I sue the tracking company for losing my package?

I sold something on eBay and they wanted to return it. When they returned it, they shipped it through FedEx. A direct signature was required. FedEx says they delivered it and it was signed for by msthefen. I do not know anyone by that name. My husband and I are the only ones who live here. I have my lease to prove it. I called FedEx and they said they will investigate. The person assigned to the investigation called me a few days later and said she is deeming the package as lost. I filed a claim with FedEx and they denied it saying it was delivered and signed for. I do not know who signed for it. It was an expensive saxophone from Italy. It cost $850. Can I sue them for losing my package and not reimbursing me for it?