Most people list gems under their vendor value. I can buy 40-50 gems under vendor value, and sell them to make a profit of 10-15g. Its not a great way to farm gold, only a little thing you can do to make a little extra every day.
http://www.thottbot.com/?s=draenite#Items
Those are the most common gems undersold. I've only done this on a few servers (Maelstrom, Nathrezim, Balnazzar, Frostwolf) but its worked on all of them.
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