I hear this all the time: "Every time I post anything, someone undercuts me within minutes." Luckily, there's a relatively simple solution to dealing with undercutters: ignore them. You don't ignore them hoping they go away, you ignore them because they aren't actually going to prevent you from selling your auctions.
Think back to the last time you bought something from the Auction House; let's say an enchant. You search for the best enchant you can put onto the gear. If the lowest price is affordable, you buy it. If it feels too expensive (compared to what you've paid before or what you know the materials cost), you might buy the mats and ask friends or trade chat for someone to make it for you. You might instead look at the second best enchant for the gear if it's something you won't be wearing for long or you're not expected to always use best-in-slot enchants.