A little bit of luck, and a little bit of smart selling and buying. Through my normal farming and leveling, i found 4 helion crossbows with over 1k dps and with crit chance or a socket. All of those sold for big cash. One of the helions i sold for 70 mil in the ah. Both of my bows i found by combing the forums and bought them (relatively) cheap in comparison to what they were worth. One of the bows i found and bought on the forums for 30 million. Since it had ias, i fliped it on the AH prior to the nerf for 85 million.
Quick advice for buying and trading big cash items: Find them and buy them by searching through the forums, and sell them through the ah. People who sell on the forums are often not patient enough to wait 36 hours. And since there is no transaction fee, you will often be able to negotiate items for a big discount. When selling on the ah, the starting bid should be reasonable, but you should always set the buyout at an extravagant price. You'll get a lot of exposure for your item on the AH and there's a chance someone with deep pockets will see your item and impulse buy it even at the inflated buyout price.