A common scam that is being operated across car and bike selling websites at the moment.

Fraudulent sellers are advertising vehicles for sale at well below market value in order to entice would-be buyers to send them money for items that they don’t own.  Typically these vehicles are being advertised online by the genuine owner and the ads are simply copied and listed for sale at a reduced price on another website such as Trading Post or bikesales etc.
 
The criminals only correspond with interested buyers by email as they do not reside in Australia and the contact number provided in the ad is usually disconnected or incorrect. The seller attaches a sense of urgency to the sale (e.g. divorce, moving overseas) and once they determine a sale is imminent, they forward the buyer a fake invoice which appears to originate from either another auction website with instructions to make payment via Western Union or Ebay along with advice that the purchase is protected by funds that the seller has saved in a fake buyer protection program.  The buyer is informed that once the payment is received, the vehicle will be shipped to them and, should the buyer change their mind, their money will be returned to them via monies stored in the buyer protection program; the entire scam is performed without the buyer having ever viewed the vehicle or spoken to the seller. 

Ensure you speak to sellers before buying and view the item if possible. Payment should not be made until you have verified the seller.