I haven't checked WA but, having purchased 3 cars from Melb in the past few years, there is no issue for the seller to leave the plates on and for you to hand in the plates when you register the car in WA, which -- in other states -- must be within 30 days of its arrival in the state (very often honoured in the breach). You can also claim the unused rego back from VicRoads.
In the ACT, unregistered vehicle permits are a complete PiTA because while you can get them for 7 days, they're "point-to-point" permits, ie, if you have to go to one location for an inspection and there are problems, you'll need another permit to take the car to the rectification workshop...and possibly a third if re-inspection is required. So, it's much better to get the car still registered in Vic with plates on. Ask the seller to delay advising disposal to VicRoads until you are all ready to transfer the rego in WA.
In the ACT, there is also a mandatory vehicle ID inspection for interstate vehicles that are less than 10 years old. Again, check WA requirements. If this is needed, then you'll also want to be in possession of the original certificate of registration with completed copy of notice of disposal by the seller, as well a Bill of Sale detailing the vehicle VIN, engine number, seller's identity and address including licence number and all your details as purchaser. This is all to prevent "re-birthing" apparently....🙄
Hope this helps