I have been looking for the achreon recipe in the auction house for the last week, and i have not found it at any price. Something must have changed since it is a very popular sword. Also two off my other guild members are having the same problem. I fear if i ever do find it the price could be huge. Please look into this issue.
I got (and learned) an Acheron recipe recently. I think the trouble with finding it is that the market for 5* recipes is very, very slow since most 5* players will just grind basil to find the recipes they want, or already acquired them ages ago and don't need to look in the AH to buy them. As such, it can be quite risky for a seller to try to flip 5* recipes in the auction house.
I was stuck with an unsellable Vog Cub Cap recipe for a few weeks, for example.
But there is hope: You can post to the stickied "Recipes Wanted" thread in the Bazaar subforum and with a bit of luck someone will help you out.