[UPDATE]: Auction House Servers are no longer online, making this achievement no longer attainable.
[original solution below]
The best way I found to do this quick and easily is when I was already playing online with a buddy. I told him that if he bought 5 cars from me that I would then buy them back from him. We would then both get the achievement at the same time. He agreed, and it worked as we thought it would.
(Auction any 5 cars you want)
-Set selling price to minimum (5,000).
-Set Buyout to Yes.
-Set Buyout price to minimum (10,000).
-Do this for 5 cars.
Your friend will need to know some minimum info in order to search for your cars that are for sale:
-Make (if you know it)
-Year (if applicable. it helps narrow it down)
also tell him to set price when searching to "affordable".
After he buys your cars, have him go to the "My Bids" tab of the auction house so he can actually select "receive car" to complete each transaction.
YOU will need to then go the "My Auctions" tab of the auction house to select "receive money" for each transaction.
You have now sold 5 cars and he has bought 5 cars. Switch to him selling those cars and you buying them following the above steps and you'll both have the achievement.
Screw auctions, "buyout" is where it's at! :)