Another thing you may need to look into is a trip permit. If the seller doesn't want you to drive the car away with the original license plate you will either need to trailer the car home or get a trip permit from the dmv so if you get pulled over for no registration/tags, everything is legal. Trip permits cost $5 on the dmv website. Print it out and tape it on a window and you're good to go.
Edit: Autocorrect can kiss my ass.