I'm still leveling my eng toon, and my JC isn't even 525 yet but I've made almost 10k gold without even trying. What I've learned recently is that the trick is persistence and perseverance - if that gem doesn't sell one day, make sure you're reposting it every day. Also, you're shooting yourself in the foot if you undercut by huge margins. Make it a few silver, maybe a gold, but there is no reason to undercut by 50 gold unless you want to cut into your margins.