The achievement is self-explanatory. The Auction House is a place where you can buy other people's cars or sell your own. Buy or sell 10 in total. The easiest way to get it is to sell cars for 5k, and buy them as well. A little tip would be to get $50,000 and buy cars that are cheap and "Not Featured" (Grey Background) using the search. You often win, because not many people see this auction, making you more likely of winning it. The achievement will be given when you either "Receive Car" or "Receive Money" form the 10th car.