Do I pay sales tax on a used car from a private seller?

In many states, yes. Even when you buy from a private seller, the state typically collects sales or use tax when you title and register the car, often based on the purchase price or a standard valuation, whichever is higher. Some states exempt certain family transfers or use a flat fee instead. Keep a signed bill of sale showing the price. Your state DMV or revenue department lists exactly how private-sale tax is calculated where you live.