With auctioneer, you can set it to not list more then 'x' of a given item at once.
You can also set it to undercut by a percentage or by a value.
You can also set it to not undercut beyond a percentage of your overall value... ie, if something is worth 10g, don't undercut below 8g if someone is selling too low.

I undercut my competition by 1 copper.
And never list more then 5 dream shards/greater cosmic essences/abyss crystals at once, or more then 40 infinite dust.

And then just log on the AH toon, 3-4 times during the day. Or whatever...
And hit "batch post" each time.
Which lists anything, to bring you back up to the total of 5x whatever or 40x whatever.

There are auction addons which can take down auctions too.
Honestly, not really that into the AH game anymore, but they're an option.