I have sold Singleton postcards in the past - they are not that unusual. You are an experienced eBayer - I am sure you know how to set up a favourite search so eBay will email you whenever a new listing occurs. Singleton in Collectables - Paper Postcards should do it; if you restrict it to Australia only you will probably eliminate the Singleton UK postcards. I would not put Singleton NSW myself - if sellers are stretched for space in the title they will often leave out the state if they think it is obvious. Good luck.
PS If you are looking for Singleton postmarks / cancellations - which is not clear from your message - make your search to include the description as well as the title. Some sellers will mention the cancellation in the description.
