LooseChange said..
No GST on private sales, providing the transaction is handled wholly by the buyer and the seller without the intervention of lawyers and the like. If there are any government charges after the sale, then on those there will be a GST component eg. Stamp Duty. If the sale is handled by a used boat dealer then his price should already include GST as it is illegal to show a price without GST.
You are talking about sales within Australia aren't you?? There is GST on the brokerage fee which the seller should be paying. No agent, no GST.
Importing a yacht is a different thing. I believe GST is charged on importation of the yacht based on what it is worth in the Australian market and Australian Customs ( as bent a bunch that God ever put breath into) would require a survey to be done (by one of their mates) to assess it's value.
Australian Customs is where the police sent their bent pennies. Didn't you watch Underbelly - Tale of Two Cities??