If you need to get a bill of sale, just go down to the DMV and they'll print out a blank official one for you. When you're selling a car, you should have the title too but it's best if you have both a title and a bill of sale. Found that out with my Z...should you get stopped on the way home with the car or before you have gotten around to getting a tag on it, the cop will want to see the bill of sale to prove that it's actually your car. Yes, even if the title is in your name and dated for within 30 days of when you're pulled over.

That's in cobb county at least, I assume it'd be the same anywhere in GA.