Flipping gems is technically profitable, but the margins are razor thin due to that 15% required price hike.
Pretty much this. I can make every type of gem in the game, and assuming you purchase the necessary items from the AH rather than farming them yourself, you'll make anywhere from 2000-40000 gold off gems after the fees (depending on the type of gem and when you purchased the materials and when you sold the gem). Now, granted, if you can farm the books & gems yourself, it's more profitable than selling the materials on the AH (due to the 15% fee). But in the end, you're better off just farming gear and hoping you get a good drop.
Anything preventing people from selling in person? Even 1% off a gem price is a good deal
You can use the trade channel if you want. I've offered my service before, but I've only ever had 2 people willing to buy, even though they'd save anywhere from 200k-2 million off the gem should they go the AH route (Whereas I've sold plenty of gems on the AH).
I mean, I guess I can understand why for the more expensive gems, because I can't possibly afford to make a perfect star nor radiant star without the money upfront, but with even the flawless stars it's like that.