If you are the buyer, offer the salesperson 15 percent to 20 percent less than what you can really afford. “That way, you’ll be leaving room for concessions,” Larrick says. For instance, if you absolutely can’t spend more than $6,000 on a used car advertised at $7,000, try offering $5,100 (15 percent less than $6,000). Next: Explain why this price is reasonable (the car has a limited warranty or few of the extras you were hoping for). Reverse logic follows if you’re the seller: Make the initial price 15 percent more than what you’d accept — if you must get $6,000 for your car, offer it for around $7,000 — and have reasons why. Remember: Set a realistic limit and have a plan B if you don’t get a deal.