Everything I sell so far are items I've gathered myself while leveling a job so it all becomes profit for time spent acquiring a higher goal. To me, anything valued on the market that is more than what I would get by npc'ing it. Is profit. In that regard, I will undercut depending on how much is currently listed. And how often it has been selling. I won't under cut if I don't have to, but if I do, it will be enough to get my item sold. What I won't do is play item tag by undercutting by 1 gil. I usually cut by 5-10%.