The small business CGT concessions can reduce or wipe out the capital gains tax on selling your business altogether, but only if you qualify, and qualifying is more involved than most sellers expect.
Technically, they are four separate concessions under Division 152 of the Income Tax Assessment Act 1997 (Cth): the 15-year exemption, the 50% active asset reduction, the retirement exemption (a lifetime limit of $500,000 per individual), and the small business rollover.
To qualify at all, you must satisfy either a $2M aggregated turnover test or a $6M maximum net asset value test, on top of asset-specific requirements including the active asset test. Each of the four concessions then carries its own additional conditions, and passing the entry tests does not guarantee any particular concession actually applies to your situation.
What you as a seller should ask your tax adviser: which of the four concessions you actually qualify for and in what order to apply them, well before you sign anything, since the eligibility assessment can affect how the deal itself should be structured.