This trick will work every time when there's a unstable landblock or instance.
1) Get one of your other character (or your friend's character) to sell something on AH. For example, a stack of Linen cloth. Give it a 100G buyout (or whatever value you want to dupe)
2) Get your character to Buy the item as per normal. The seller will get the money, and your money will be deducted.
3) Next thing is that you need to get to an broken landblock or instance as quickly as possible. Broken landblocks are places with no monster spawning (where normally they would), places that will get you stuck in looting/mining/skining mode until you quit your client or get disconnected from server from timeout etc.
4) Once you get into the bad landblock, and run around a little in there, you will eventually get disconnected from the server.
5) Here come the best part. When you reconnect back to the server, you will get rollback to the last save state. Most of the time, if you are able to get to the bad landblock quick enough, your last save state will be at the point before you spend the money to buy the item -- which mean, you will get back all the money from the purchase you had a while ago.
Tips on how to execute this:
1) Scan Chat Channel and Realm Forum to find out if any part of the world is broken.
2) If you have Warlock friends, get them as close to the bad landblock as possible without getting suck into it (otherwise they will probably freeze up and need to relogin).
3) Otherwise, you have to use the usual transporation available to you to get to the broken landblock as quickly as possible. You can make use of the teleportation client if you are brave enough though.
4) Once you are ready, setup the Buy and Sell thingie on AH. Upon buying out the item, immediately get to the broken landblock. If you have the warlock friend, ask him and his fren to summon you close to the bad landblock, and once you are there, run into the bad landblock.
5) Rinse and repeat, and exploit server instability for profit.
Risk Involved
1) Obviously, if you get rich too fast and too quickly, you risk getting investigated and banned.
2) If you can't get to the bad landblock in time, the server may have save your character data, and you will be rollback to the state where you have already spent your money. If this happened, all you lost is the small percentage of AH commission fee from the completed sales transaction. So just absorb the losses, and try again when the bad landblock is more accessible to you

Here is wha catch here.
U got like 10 seconds to get to place and somhow get disconeted (like when u see server restart than do it the best time to do)

BTW something to add: download while u do this bounch of files so u make major lagg with lag u get lot of time insted of just 10 secs.

I descovered this while i was buying from AH i bought some item and server restarted dunno why i logged in back (cus of funserver the word was in about 25k gold) i saw my item and my 190gold ...i shoulod have 165k cus i bought that item but still i had 190k xD
Hope it helps