It depends on the states involved.
As a resident of Montana, I can drive a newly purchased vehicle with just the title and/or Bill Of Sale for 40 days after the purchase date.
If you buy a vehicle in Montana, the seller will keep his license plates.
If you buy a vehicle in California, the license plates will stay on the vehicle until it is registered in another state. It is the seller's responsibility to notify the State that the vehicle has been sold
It is the buyer's responsibility to find out the rules in the states where the transaction will take place.
