Ok now this is no exploit but a good trick that won't get you banned.
1. First off get the item you want to sell (make sure there are few of em on AH).
2. Then put it on the auction house for a high amount.
3. Go on alt post WTB x item for 30-40% more gold then your item on ah
4. Some one will quickly try buy your item on AH to resell to you.
5. They buy it, you then say "Sorry some one just traded me the item man".
6. You now have quick easy gold, but its not over because now this person wants to sell your item.
7. So you simply if your smart quickly re post that same item, but now really cheap.
8. He puts on cheap, you buy it cancel your auctions and BOOM fooled em twice!