First off, I'd like to point out this scam will not make you instantly rich, at most you'll usually be making 80g or so from each scam but they go quick!
Requirements -
1. A Rare+ item that you is selling for 10-40g, but there must only be one of this item on the AH or it won't work
Steps -
1. Send about 100g to a alt character, you don't want your mains name being trashed on the server.
2. Buy a rare quality+ item that is cheap (1-40g) and make sure there are only one of this item on the AH (i.e don't buy Destiny if there are 4 of them on the AH)
3. Put the item back on the AH for a higher price (I do about 90g but you can do any price you want, I find as the price goes up people are much less likely to buy the item for you)
4. Type /who 80 Orgrimmar and turn on guilds, don't try to scam people in top end guilds because they're much less likely to fall for this.
5. Whisper random 80s asking them "Want to make a quick 50g or so (price can be whatever you want)" most of the time they'll reply "Sure what you need?" or i've had people just not answer or say no, if they say no just move on.
6. Tell them to go to the AH and see if there is [insert the item you bought earlier here] on the AH and how much the lowest one is selling for, they'll say yes and [insert price it is selling for here]
7. Next say "Heres the deal, you buy the item and go to the Booty Bay AH and sell it there for [insert a higher price than you're selling it for here] and i'll buy it on my alliance character quickly, he's parked there"
8. If they say sure tell them "Tell me when you've bought it so I can log on my alliance and get ready to buy the item"
9. They'll whisper you saying they've bought the item and bam, you've sold a item for 2-10x the price it's worth.
So far I've made 1k gold in a few hours doing this, I just needed money for my dual spec but if I wanted to keep going it'd be easy.
This scam is risk free because you're just getting them to buy a item from you, they are trading their gold for your items so it isn't listed as a scam in Blizzards eyes.