For epic and meta northrend gems, I have a 'buy price' and a 'sell price' per colour. Both the buy and sell price drift according to supply and demand. When I am low on a colour, I pay more and charge more. When I have too many of a colour, I pay less and charge less. In either instance, I move gems up or down in a 5-10g range. My sell price is currently based on a range between MaximumBuyPrice + SmallMarkup, and MaximumBuyPrice + LargeMarkup. I list 2 gems at a time. My prefferred stock levels are between 40 & 80 uncut gems. I do not post if I can't make my minium sell price. I have most cuts, missing the many of the Hit +
For example - Eye of Zul
At the moment I am buying all Eye's of Zul for 80g. My minimum 'profit' is 10g, and then I add the AH cut (5%), giving a minimum sell price of roughly 95g. My maximum sell price includes a profit of about 60g, so (80+60) *1.05 is rougly 150g. I use QA3 with a group containing all Eyes of zul (including uncut) post 2, no auto fallback, 95g-150g.
Gems are very cyclical in nature. Often someone dumps 30 cheap uncut gems, then supply dries up for a fortnight. A crafter dumps a cut below my price range, but leaves other cuts alone. If they dump gems below my uncut buy price I will often buy them out. I am currently selling 20-30 gems/day, more on the weekend. The problem with my current strategy is that it is too simple. I post the same number of popular and unpopular gems. I miss out on sales with not enough stock on the AH. It redeems itself by being very quick.
I use auctioneer with a snatch list to buy gems. I simply can not get enough gems from the AH at a price I am willing to pay. I also dislike the time commitments from advertising in trade. My solution is that I have a guildy 'farm' the commumity for me. He provides alchemists with mats, pays them a pittance, takes a cut and sells gems to me. This still is not providing enough gems. So now my guildy advertises in trade buying gems, and on-sells them to me at a small but guaranteed profit. This latest approach is solving my supply issues.