Undercuting becomes a problem on slow selling high budget items. Where you can se usually a few listings like 2 or 3 and just a few more on history. Usually high end crafters on this case tends to match the price of the first seller, at least the ones with "etiquette" so the one who sells becomes completely random on about who is the buyer gonna pick since they are all at the same price. Problem with undercut is:
1.- Player new to the market gets into it and forces a crash from lets say, 5.000.000 to 2.000.000. Of course you could "flip" it, but on slow items that flip could give you rewards a few weeks later.
2.- Player new to the market sells the item cheaper than the materials. Let say an ifrit i95 weapon cost 2.000.000 millions to make, rounding the ifrit horn to 1 mill, the BCIII to 1 mill and giving away the other materials. New player puts the ifrit weapon at 1.700.000.
The reasons for the second one are usually the wrong thought of "if i got it myself (retainer, drop, gathering) so is free cause i didn`t invest any money and i can undercut as much as i want cause others spend money". Well, no, there isn`t such a thing as a "free" item on the economy, even if you didn`t spend a single gil on the items, they still have value, and use those materials to craft something is still an investment. And in both escenarios all players are losing money, cause the undercutter is getting less money than he could have by sticking to the average prices or by selling lower than the materials price, and the other players lose their chance to sell that item, which is a loss for everyone selling.
Now about the undercutting itself, is normal, yet it could be controled by a few market restrictions and options that are most needed, in fact the whole MB could use some serious enhancements beyond the listing and the interface.
-A limit on how much you could undercut someone, or at least, a warning before listing at +15% under the cheapest item that warns you are about to damage the market.
-A tool that allows MB players to know the average price of the items, so you don`t have people undercuting without proper information, history of sells just doesnt make it, in fact is natural to see jumps and drops of 500% on several items all over the place.
-Buy orders. Seriously. Players don`t have a way to know what players efectively want (demanding sucks, i play on cactuar, highly populated server and demanded items are so low, barely above 100... obviusly people doesnt care about demaning) and how much they are willing to pay.
That alone would stabilize the markets and stop the cobalt halberd at 6.000 gil, next day, 54.000 gil. And that repeats all over the place, and as time goes on, more and more players get to the 3 star markets and things just get worse and worse. The devaluation is normal, what is not normal is see how augmented saurian items for example, are being undercut even up to 2 times less than their actual material costs, which includes all the materials of the normal tabard, which is, just fun. The solution to undercutting is not just flipping. Undercutting is normal and a form of competition on properly regulated markets, on a absolutely free market with just a 5% fee for the gil sink, undercutting is actually hurting us all and in fact, making fair competition almost impossible.
You can`t regulate human "specialness", but you can regulate the markets.