License plates may only be used on one motor vehicle at a time, but the transfer of registered plates from one car to another car purchased by the same owner is a legal transaction. Regulations and specific procedures vary between states, but as long as both automobiles are registered under the same name and are the same classification of motor vehicle, license plates may be transferred at the local DMV for a small fee.
Drivers must go to the DMV, fill out a License/Registration transfer form and pay the license plate transfer fee along with the new registration fee if it is applicable in that state. Many states allow drivers to simply renew their previously paid registration fee for a lower price.
The level of stringency in this process varies widely from state to state. For example, New Jersey has a relatively straightforward law that allows for the transfer of license plates from car to car, provided the cars are owned by the same person, but Oregon and Pennsylvania have more complicated transfer laws that depend upon several specific circumstances in the transfer situation. The information kiosk at the local DMV and the American Automobile Association's Digest of Motor Laws are both valuable resources to consult when dealing with the license plate transfer process.