If it's an automatic and it jolts into gear when you accelerate evenly, the transmission is probably bad. You just hop on out of that car, spare a couple of polite middle fingers for the nice salesman, and run to Mexico and sweet freedom. That is not fixable. Nobody understands transmissions -- not even a mechanic. All anybody can do about a transmission is just try to keep it happy, and try to shield our loved ones from its horrible wrath if we fail. Watch the oil pressure, battery and temperature gauges -- you probably won't be out long enough to trigger them, but they're your last resort. If something is so wrong that the gauges start maxing out during a short test drive, that car is about to transform into Shittimus Prime, the shit robot, who will shit all over your life until you are dead.
After you've done your due diligence, cussed and punched your way through a few dealerships, combed Craigslist and found it to be full of potential rapists, and attempted to appraise the integrity of every single system in a giant, complicated machine that you've probably never seen before -- you're finally ready.
You're ready to go home.
Oh, not with a quality used vehicle -- nobody promised you that. If you're smart, and you've followed this guide to the letter, you've just realized that every single vehicle for sale is an utter piece of garbage; overpriced and fundamentally broken, guarded by a zealous tribe of conmen who would rather chop off their own junk than come within spitting distance of a blue book. No, the only intelligent thing to do now is to leave with your dignity intact. Don't forget to turn dramatically at the exit and promise that they'll rue something -- almost anything will do: "The moment you met me," "the decision to cross me," or even just the boilerplate "this day" are all fine, as long as they know they'll be ruing something. Don't worry; it's all part of the dance.
It just gives them something to remember you by, because you'll be back here doing this again the next time somebody gropes you on the bus.
Buy Robert's stunning, transcendental, orgasmic science fiction novel, Rx: A Tale of Electronegativity, right here. Or buy Robert's other (pretty OK) book, Everything Is Going to Kill Everybody: The Terrifyingly Real Ways the World Wants You Dead. Follow him on Tumblr, Twitter and Facebook.