There's pretty much NEVER any good reason to get angry at people who undercut by a wide margin. If they do it once, then it's just a blip in the market history; it will sell fast, and prices will resume as they were. They may have been fools for selling so low, but by no means does the market get "tanked".

If they do it many times, then they have more stock than the current market has a demand for and so the price SHOULD go down. If you're certain that your product is worth more than they're selling for, then just wait out the storm. If you're right, his overstock will eventually sell out and prices will return to what you think is normal. If you're wrong, and the prices stay low, then the market has shifted; demand vs supply was not what you thought it was. You screwed up; own up to your mistake and price low, as the current market demands. Don't blame your competition for knowing the market better than you do.

It's the same deal if the guy is trying to "control" the market by selling cheap to force out competitors. You're the competitor in this case. LET yourself get "forced out", hold onto your product, and once the guy thinks he's in control and starts jacking prices up again, then sell. The marketboards are both like and unlike a real economy. The forces of supply and demand are in full effect, but entry into a market costs nothing. Being forced out of a market in real life has horrifying consequences due to the extreme costs involved in setting up shop. In FFXIV, though, you can join, leave, and rejoin a market quickly and easily at any time.

Honestly, the only folks worth getting mad at are the ones who MINUTELY undercut by one gil, or ten gil, or whatever. These folks are basically cheating the system; they aren't pricing cheaper in order to keep the markets competitive, because really, their goods AREN'T cheaper to any meaningful degree. They are pricing cheaper to keep their names visible on the first page of the price list.