Howdy Charlie. Sounds like you are doing what you can to ensure the trike is kept in good running condition. It's a real hard choice when it comes time to give up riding. Don't know if I could make that decision just yet. If you don't need the money selling the trike would bring and you really think there is a chance you will want to ride more then I'd give it a while and see what happens. Getting back into trike ownership would be a lot more expensive then holding on to what you have for another summer...
As for where to advertise I think you are correct. Craig's List and American Classified (Thrifty Nickel) have wider coverage so you will get the better coverage. If you use American Classified see if they are still offering the run until it sells deal. Several months ago I sold my Class C motor home that way and it worked out well..
I wish you luck with your choice and hope you will succeed on that journey.