Nacho, I ran some tests and chances are, you were in the Real Money Auction House instead of the Gold Auction House.
Here is a screenshot of me searching for Subtle Essence at the Gold Auction House.
Now, here is a screenshot of me searching for Fallen Tooth at the Gold Auction House.
As you can see, both of these searches showed the items. So Blizzard did not forbid it due to how non-profitable it is.
Next, I'm going to see if I can see my auctions for these items after putting it on sale at the Gold Auction House. Below, you can see that they do show up.
Finally, I'm going to switch to the Real Money Auction House to see if Fallen Tooth and Subtle Essence show up.
So hopefully you were just searching for those items in the wrong place. If you were in the Gold Auction House, then I would have to do some additional research to see what is wrong.
TL;DR Switch to the Gold Auction House.