Dave, you need a current title, or if there isn't one(some states don't require a title if the car is over 10 years old) a current registration for the car you want to buy..
And besure to check the VIN #..........
On the car and any paperwork.
I know that if you buy a car in cali and it's an out-of state car... they want it registered by YOU in the original state.... before you can get tags... at least that is way way it worked for me ..
DMVs pretty much suck everywhere though... IMHO
Some states require a bill of sale, some consider the title a bill of sale. Research your local DMV, they will be able to answer all of these questions for you. In Virginia, all this kind of information is available on the DMV web site.
In NH you need a title if the car is less than 10 years old. Ten years on you only need a bill of sale. I kept the original Utah tittle to mine because I got a Bill of Sale when I bought it in Utah. I used the Bill of Sale to register it when I got it home and kept the title.