The market's a big place. Even within a single category (food, in this case), it's unlikely that he's messing with ALL of the prices. For the time being, tap the markets he's not bothering with. If he's selling at a loss, he'll eventually believe that he's "scared" everyone else away and then raise his prices. Watch for that, and when it happens move back in.

If he does farm/gather his own ingredients, though, prices may stay where they are. If that's the case, then you'll either need to abandon the market completely, or farm/gather your own ingredients as well. That's a possibility that ANY crafter who doesn't level DoL jobs always has to worry about - that their market will become dominated by someone who can sell cheap because they don't buy the ingredients. If you haven't leveled DoL because you don't have the spare time - well, MMOs have ALWAYS favored players with more free time. That's just a fact of life.