What should I say when making an offer on a car?

Be clear and specific: name the vehicle by VIN or stock number, state your offer as an out-the-door price including tax and fees, mention whether you're financing, paying cash or trading in, and say how long the offer stands. Briefly explain your reasoning, such as comparable listings. Keep it polite and in writing. If you're pre-approved, mention it, since it signals you're ready to buy.