Win an auction by outbidding someone.
This achievement is actually harder than it seems because you can never know whether someone else is going to bid on a car you want. You could get lucky and randomly win one, but the easiest way to get this would be to boost it by finding someone in the Achievement Trading Thread to auction a car that no one else wants and placing a very high starting price on it. Bid once on the car and periodically check back to see if anyone else has bid on it. If someone else has, outbid them so you can win. Once the time runs out and you obtain the car, the achievement will unlock.