In most states where you're buying the car, you're required to get a temporary registration, good for 10-30 days, which will enable you to drive from there to your home state, where you'll then need to get the title transferred and the vehicle permanently registered. The temporary registration is usually very inexpensive. In some states it consists of a cardboard temporary plate which goes in the license plate holder; in others it's a sticker that goes onto the rear window.
If you don't get a temporary registration, you may not get pulled over on the way, and if you are you may or may not get cited for a lack of registration. But I wouldn't risk it. It's easy enough to stop by the local DMV office with the seller, to pick up the temporary registration.
You should NOT use the seller's plates, which belong to the seller and which he may wish to transfer to another vehicle. If you use the seller's plates and you then get pulled over, you may be putting yourself in a difficult situation, even if you have the signed vehicle title with you. Don't do it!
Most state DMV websites are very informative and have all the information you need to proceed. For example, the Oregon DMV website tells you what you'll need to title and register the car bought out of state (click
here). In which state are you buying a car?