CASH... as much as humanly possible. Bill of Sales are good too, so nobody can argue. Make sure seller signs title. They have to deal with the taxes.

DON'T TRUST ANYONE.. i have a quick story for you guys. it was my dad trying to sell his Mustang to some guy.

Guy comes over and wants to buy the mustang, they agree on price and the guy whips out his checkbook.

My dad: whoa.. what are you doing?
Buyer: writing you a check
Dad: um..i dont think so
Buyer: what, i have the money in my account.
Dad: i don't konw you from Jack. I'm supposed to trust you believe you that this check is gonna be good when i take this to the bank? How do i know you wont stop the check?
Buyer: I wouldn't do that, i'm good for it.
Dad: Well then, you can take the car tonight, but im keeping the title and a set of keys if this check doesn't clear.
Buyer (getting a little upset): you don't have to do that, im good for it
Dad: I'm not giving you a car, if youre gonna give me a piece of paper.
Buyer: Ok, ok.. thats fine.. take the check and call us when the money clears.

The buyer took the car that night. They wrote some stuff (bill of sale/contract about the title/money clearing). couple days later, check cleared and my dad signed the title over

Make sure you have the cash. I know it sounds like my dad was a dick, but if you dont do some kind of paper trail you will be SOL. good luck man