Go to theatre 45 min before show. Wave ticket in air while walking back and forth on sidewalk. Or find cancellation line and do the waving there. Someone will approach you asking if you are selling. Sell for the price you paid.

You can list a physical ticket on Ticketmaster resale by taking a photo of the ticket or entering the barcode information. If that doesn't work you should be able to sell it at the theatre day of.