Check the numbers.
Credit: Gabriel Bouys / AFP / Getty Images

Allen always looks for what are known as "numbers-matching" cars: That is, cars where the engine, transmission, and rear axle all link up to the vehicle's VIN number. "True investment cars are going to be numbers-matching cars," he says. How do you know if the numbers add up? Allen says most motors have the last six digits of the VIN number stamped on them, which makes that an easy check. The transmission and rear end are a bit trickier. Usually they'll be stamped with date codes, which you can look up to determine if the dates sync correctly. "It's not a problem [if not everything matches]," says Allen. "It's just not going to bring the bigger money that an all-matching car will. You can never lose if you find an all-numbers-matching car. It's a way safer bet."