Undercutting is a mechanism of competition as was mentioned before.
Thinking it should be a bannable offense though just because it's irritating to some is simply ridiculous. Look at the market from the consumers/buyers perspective. People that sell stuff for large sums are the irritating ones. So now we can't sell for low, nor can we sell for high...and whatever price you sell whatever at, it'll fit either of these.
In real life there are regulations that prohibit severe undercutting...yes. But they have a very important reason. Limited resources. You can't have any income by selling items for more than it cost you to make them, so the absolute lowest you can go en masse is to even out your costs to make em (or give for "free", which is effectively charity and not sale). If you sell for less than it did cost you to make, then it's either to get rid of something that failed to sell for a LONG time (which is acceptable) or try to destroy your small-time competition (which is NOT acceptable) with your overwhelming budget. As such, with real-life situations, it is very easy to prove who had malicious intent.
In game, the cost of making anything is zero (since you can get every single thing in this game without spending a single gil). You cannot possibly sell something for less than it did cost you (or rather, how it COULD cost you), since you can't sell for negative...as in you paying others to take it.
In other words, no. There is zero reason to complain about people undercutting, as there is zero reason for people driving the prices up. In the end, it is the supply and demand, supported by inflation, that will bring the general price to what the item is "worth", per see. In other words, the only thing Square Enix SHOULD do is control the inflation by money sinks and careful implementation of gil generation. Everything else WILL work itself out if this one thing will be properly done.