There's nothing wrong with undercutting. WoW is an open market and you are free to list items for whatever price you want. If someone chooses to grossly undercut then it gives others the opportunity to buy them out and relist for more.
Undercutting by like 1c is just common sense, you have the cheaper item you have the faster sales.
The only issue I have is AH campers that relist items like every 5 minutes.