Forza Motorsport 3

Best way to do this is to get a good amount of money and in the auction house make shore when you search you set it to search for
Class e or lower w.e
and car level 0


then look for the ones that are going for 1 min

now what iv spotted is that whenever some one bids the time clicks back up to one minute so your going to have to hope that people give up

Be sure that you don't utilize the buyout option, as this does NOT net you this achievement. Look for the cars that are going within the next minute and bid, just keep refreshing and hope the others don't spot that they've been outbid in time. Also, make sure that you go and finalize your deal, I think the option is called resolve auction or something to that effect.

Aim for the lowest price but the lowest time so you are not out of pocket but get the achievement quickly. Mine was a Lambo Reventon for 5,000!! I sold it after for the same price so i got the selling a car in the auction house as well

While everyone else's solutions will help you get the achievement, I think I have a better way of filtering through the cars to get you a cheap one faster. Rather than limiting your search to a specific rank, the only filter you need to turn on is price. Set price to <30,000 CR, or whatever you feel comfortable paying, and press X to search. You might be surprised to find better cars going for less than crappier ones. The game seems to auto sort the cars found matching your search criteria by ending soonest to a degree. You may need to scroll down for a second to find the ones actually ending soonest, but it won't take long. Remember when bidding, if you don't care about the car, you can always sell it back for the "Auctioneer" achievement (unless the car has a locked setup, the one I won did ), and because you outbid someone else, you have an idea of what the car may go for.

Because the game is getting older, there are less cars being auctioned. There are still plenty, but you may have to wait a few hours for the next available car's time to end. I find myself having to wait as I type this. The next auctions ending with other people bidding on them (don't forget to look and see if other people have bid before pledging your money) is 25 min. away. I went ahead and bid on 2. One's a C class and one's a B class, both are pretty cheap. Keep in mind, you don't have to swoop in at the last minute. Bid and go do something else. So long as you outbid someone else and won, the achievement will be yours.