Do you have to pay sales tax when purchasing a used car from a private party seller in California?

No you do not. Sales tax is what businesses use to pay to the state. It's the "state's share" on a taxable item. A private party seller is not a business, so legally they cannot charge a sales tax. For example they cannot sell a car for $2,000 plus tax. However, you WILL have to pay a use tax when you go to the DMV to register your vehicle. The use tax is basically the same exact rate as sales tax. The state will tax you anyway they can.