Anyone selling stuff for less than NPC prices is, um, "unclear on the concept". It wastes their MB slots and anyone can buy it and resell to an NPC for a profit. Pro-tip: The original seller would get more, and instantly, by NPC'ing it... and the NPC price is shown on the item during the process of placing it on the MB(!).

As for other "undercutting", it is kind of "crazy-making" to see someone list their items at less than the currently-listed items. But understand two fundamental principles:
1. The item probably did not cost the player actual gil, so it's seen as 100% profit, no matter what the price. and
2. Many people (like me!) put stuff up at the current listed/sold prices, but come back half a day or a day later to find it unsold and a history of many sales at greatly lower pricing. Now refer to principle #1 above.

Early on in the game, when demand was high and supply was low, crafters & gatherers could get away with charging even 100x what it cost to make, and there would be a buyer. That time is gone, though it will recur at each new content during initial high scarcity. (Just look at Blue Fox Hide and BlueBird Minion pricing for current examples.)