im looking at buying a low km AU (4spd auto) gearbox.

its gonna cost me $750 (from a wrecked falcon). the price is supply (im fitting it myself)

is there any things i should look out for or ask when buying it?

is it normal to have a warranty, even if its limited?

im buying it from a legit Perth wrecker.

Yeah look out for one at a more realistic price. Unless that's got a warretny then they've seen you comming. $250.00 is about the price range for the BTR boxes second hand in unknown condition.

Look for how worn the extension housing slip-yoke bush is, by puting a slip-yoke in it and check how much wobble (up & down) it has. Take the bung out and see if it has ATF in it, and check the condition of it.
Its always "Buyer be-ware"