If you are holding physical or "paper" tickets for an upcoming event and can no longer attend:
You can often transfer tickets to another person through your account, though this is now almost exclusively a digital process. Options for Unwanted Paper Tickets ticketmaster buy back
You can list physical tickets on secondary marketplaces like StubHub or SeatGeek , though you will be responsible for shipping the physical paper to the buyer. If you are holding physical or "paper" tickets