Never, never, ever pay cash to anyone for large sums of money such as a car. Always pay with a cashiers check or money order. Leave a paper trail so you have proof you paid the full amount. Pay cash and its the sellers word against yours and they may say you didn't pay the full amount when you did in order to try and scam a few more dollars from you.
Again, never pay cash for bigger deals. And get a bill of sale along with the title once you pay him in full.
And as the others above said, get a loan from a bank. Much, much safer. Have the bank make the check to the sellers name in the full amount so you can be done and over with him. The only obligation you have with payments is your bank and thats much safer and trustworthy.