When you look at the info window for a crafted item there will be a signature in the bottom left corner. That is (with some strange exceptions) the name of the crafter. If there are no signature then the item is a drop (most likely from a retainer doing quick exploration). Undercutting items that weren't crafted by a player is only driving the prices down. Let them sell, that player won't relist anytime soon, as he/she doesn't make it himself/herself. The reason the prices on so many items are so low is because your guys are fighting windmills, and that's not very productive. You have expenses to think about, not so those that sell items gotten by retainers. Every now and again it seems SE is changing the loot table for the retainers, so after a patch (minor or major) there will most likely be changes. Try finding something that isn't dropped by retainers in stead. That way you can actually make some money. I normally put those items that are dropped by retainer on a relatively high price and don't start crafting them before it sells at that price AND I don't find any without signature.
If you are desperate about making money, and have time to spend, sell items sold by vendors, for gil, people will buy iron ore and what not, that you can buy cheaply from NPC's.
Another, but far riskier way, is to buy up general-purpose dye on all servers in the data group and sell it at a higher price. If the dye is in high demand you will earn money doing it, but at times you can be stuck with large quantities.
And most important of all, don't list more than 1 or 2 of the same item at a time. Listing too many is a sure way to crash the market. Selling 1 item at 100k is better than 5 at 20k, because it saves you time, lets you put up a larger assortment of items, so you might actually sell 2 or 3 different items at a high price in stead of 5 or 6 of the same at a mediocre price. And never try to make and sell something that there already are more than 2 players crafting and selling, that only ends in a trade war.