If you're constantly on the AH...undercut his 50g, wait for him to drop below your cost of mats, buy him out and cancel your lowball price so the highest is up. I had someone in the bag market that was bugging me, so that's what I wound up doing. But you have to be watching like crazy. Luckily, this guy only had one bag he kept canceling and reposting lol.
At the time, either his mat costs were below mine or he was willing to take losses to drive me out of the market. At any rate, I ended up (mostly) leaving the market, as there are far too many players by now. Making significant profits from this market would involve a lot more playtime than I have - or, at least, than I'm willing to commit to gold-making, since I also happen to enjoy playing my tools
I've followed the excellent advice given by a wise goblin in these forums : if there are too many people selling a craft, sell the comps. Most of my money these days is coming from the ore shuffle. Prospecting/crafting/disenchanting is a bit mind-numbing, but with the proper tools (eg, panda) and a lot of empty bag space, it's almost bearable.