Earlier this week I finished my auction house bot, and I've had more than a little success with it.
The script searches for items under parameters set by the user (stats, price), and refreshes its search a few times a minute. If it finds an item under those parameters, the script automatically purchases the item.
Limitations:
It cannot yet cycle through different sets of parameters (to look for different types of items.) It can search for a single type of item (say, 1200 dps one hander with 200 intelligence for under 6 million) and purchase it if it appears. In the future I'd like the script to have the option to make a noise when such an items appears rather than automatically buying it, but that's for a later development.
Since other scripters are working on the same thing, in a week or two I'll turn this over to the public domain, and playing the auction house will probably be ruined for anyone without a bot. Before then, I'm willing to hand this over to a small group of people (fewer than 20) on the condition that they cut me in on their profit for the first week or so. You can contact me via PM or on my skype, underashmissing, if you want to talk about it.
A fair warning:
This script isn't going to do much for you unless you have ~10-20 million to play with. You get out what you can put in. If you can't afford to buy a crossbow that's worth 10 million at 4, then you're not going to get much out of this.