I also love undercutters. The NPCs are always happy to buy the stuff I sell them for profit after buying it from AH.

Seriously. If you want to support a legit crafter by selling cheap materials, why don't you just do a shout and sell directly? Or support your FC by supplying materials?
If you put in on the AH the chance is high that some RMT or some other id*** will buy and resell.

Prices will regulate without you undercutting. If there is low supply prices will go up, but thanks to the history players can see what's the usual price is and just don't buy it till the price lowers. Or buy at the higher price if they really need it.

Why would I invest materials and time and all the lv50 crafting and gathering classes that took me a long time to get to sell everything for peanuts when people are willing to buy for a reasonable price?

I guess it's a matter of how much your time is worth for you.