Peter, know what you are saying, most stamps of the Australian variety are not really that rare.
For instance The Penny and Half Penny and The Shilling are quite common even today.
However if you delve deeper you just may have a special one like the Black Swan for instance, from WA.
Get that book that Bart suggested and you will be amazed at what you might find.
However if you wish to sell it as is send a PM
Leon