Tire sizes are not unique to the Slingshot. We use (relatively) common tires that should fit a variety of vehicles. I'd advertise them on your local craigslist. EJFord is right about shipping costs. To a buyer the price + shipping would most likely be higher than buying the same tires at a local store.
I was in Florida in Feb and bought 2 brand new wheels and tires (front) and slightly used rear wheel with a second new tire for $300.00. Really no resale value on Wheels and tires from the Slingshot. I was there and had space to hall them back to Northern Indiana.