Ok, i just started on this website and thought i'd put up what i do to make some quick easy cash.(with some luck)
First of all, it helps to be a miner, jewelcrafter, or both, but its not needed.
Second, you'll probably want some gold to start out with, like around 200+
Now to the fun part. Go the Auction House and search Saronite Ore.
Buy a few stacks of the cheapest that's posted, preferably under 40g.
Now go get them from the mailbox, and get a jewelcrafter to prospect all the ore. What you want are the blue gems. Scarlet Rubies are the best and sell for the most on my server. Once it's all prospected, go to the AH and post all of the blue gems for the going price at the time, and of course undercut.
If your a miner, you can mine the saronite and make a straight profit. I suggest Scholozar Basin, go in a circle around the zone, and every once in awhile go up and down the river, great way to farm saronite and some titanium if you have your epic flyer, if not, don't bother there's a TON of competition most hours of the day.
If your a jewelcrafter, obviously, you can prospect the ore yourself.
SUGGESTION: Check the going price of the multiple blue gems on your server BEFORE you buy the ore. On my server Scarlet Rubies go for 60g+, and the others 20-40g. The greens go for a LOT less, and i don't even think are really worth to post, i personally save them for alts and throw some in the guild bank for guildies.
NOTE: The gems are random that you get, sometimes you'll profit, others you won't. Sorta like gambling, but it's about a 70/30 chance win to loss.
And if your a miner, no loss ever
Other Methods: You can do this sometimes with Titansteel bars, by buying the mats and getting someone you know(or yourself) to make it into a bar and reselling it for more. This works with many different items, depending on your server prices.