There are two toons, besides myself who seem to undercut gems quite often.

I sent them both whispers, and one of them has agreed to match my price (and I'll match his) if we are the lowest priced gem sellers currently.
It doesn't hurt to talk with your competition.
And I'm really lucky this is the guy who was cooperative, as he's on much more so then the other guy.

The other guy is added to my friends list on the toon who most often leads each of my teams.
Whenever he is online, and in a zone with an auction house...
I know it's a good idea to check out shortly, to re-undercut him.