Hi Marx
Of course, if you're selling via a Lease/Option (Rent To Own) no stamp duty is payable until the Option is exercised and the resulting contract of sale is exchanged (in NSW).
If you are selling via an Installment Sales Contract (Wrap), your wrapee has to pay stamp duty within 90 days of exchange of the Instalment Sales Contract.
The timing of the requirement to pay stamp duty varies from State to State, e.g. in Victoria you don't have to pay the stamp duty until the Instalment Sales Contract is completed, i.e. possibly quite a few years.
Good luck.
Cheers, Paul