Here's how it works in PA.
All vehicles need titles. Title and registration are two different things. The title provides proof of ownership. Registration is the act of getting a license plate for the car in order to drive it. A car can be titled and not registered, but it cant be the other way around.
PA is very strict. As a matter of fact, in PA, you cant buy a car with a signed title and the owner not present. Both parties have to be present at the time of signing and provide proof of identity. They do this to prevent third party sales, where guy A sells the car to guy B, and signs the title. Guy B then sells the car to guy C, and the sale to guy B doesnt show up in the records. Why do they want to prevent third party sales? Because PA is greedy, and wants to make sure that they dont miss out on the sales tax from guy B. If you buy a car in PA, and the guy who is selling the car is not the same as the name on the title, you are screwed. That even holds true in cases where the original owner has died. In that case, the original owners heirs need to file for a new title in their name before the car is allowed to be sold.
If I buy a car out of state, I always make the seller go to a notary and sign a bill of sale and have it notarized. Then PA will accept it without a lot of hassle, although it can take a while to get a PA title for it.
PA sucks as far as car ownership is concerned.