roadworthy certificate valid for how long

Short answer: it depends on your state or territory.

In VIC, a roadworthy certificate is generally valid for 30 days from issue for transfer.

In NSW, the eSafety check ("pink slip") is valid for 42 days.

In QLD, safety certificates last 2 months/2,000 km for private sellers, or 3 months/1,000 km for dealers.

Elsewhere (WA, SA, NT, TAS, ACT), inspections are typically only required in specific scenarios or when requested, and any pass is usable for a short, regulator-defined window—so always check your local rules. Bureaucracy moves faster than your project car; plan your sale or rego accordingly.