Depends on the State.
I just purchased an M4 private in MD, but the car will be registered in Delaware. The DMV in Delaware told me that I can purchase "Transportation Tags" so you bring the information to the DMV and they gave me a 5 day Temp tag to bring the car to register for temporary tags. I also sold my previous car the next week and DMV told me if I trust the person that I sold the car to then I didn't need to keep the plates, but I could if I didn't trust the guy.
This website should help you out it's what I used when researching.
http://www.dmv.org/