Buying a car can be stressful, especially when you're negotiating with dealers armed with tricks designed to get you to spend more money. Wiener said that of all the negotiating mistakes that buyers make, one looms above the rest. "You never want to say what you're willing to pay per month," Wiener said. Rather than laying out how much money you're open to spending on your car payment, you should instead determine what the dealership is willing to sell you the car for. He gave the example of a client who wanted to buy a car and put $10,000 down with 0% financing.

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Persons: Tosh, Daniel Tosh, Marty Wiener, Wiener, you've, it's, David Weliver Organizations: CNBC