Disclaimer: I craft because I like crafting. I don't invest too much in 2-star crafts, although I am capable of it.

At least for the 2-star gear problem, the whole problem stems from the fact that people do not need more than 1 equipment for any part. For example, there is no point in having a Rose Gold Earring and a Hero's Earring of Healing. I won't include Allagan Earring of Healing, since it's a dungeon drop, and does not cost any resources to obtain. This means that there is absolutely no reason for me to have 2 differently overmelded Rose Gold Earrings with different stats.

Then you also consider that everyone can only craft the same item. There is no variation or something that sets the products apart. For example, Coca Cola or Pepsi? They're both cola, but they have a different flavour (last I tasted) - I find Coca Cola to be sweeter than Pepsi. You don't have this kind of variation with crafted 2-star gear. Vanya Hat of Healing crafted by me, and Vanya Hat of Healing crafted by Individual A are no different.

You can attempt to make them different by melding in materia, but due to the RNG nature of melding, you can only meld so much before you eventually have to call it quits just so you don't end up netting a loss.

Because variation is taken out of the equation, and people only need one part for their gearset, people are simply keen enough to just buy whichever is at the lowest price. And fact is, as long as mine sells, yours won't. This is why people undercut, even if by a few gil. Ultimately, crafted 2-star gear doesn't move in a fast enough volume as say materia, or crafting materials like Aldgoat Leather, Steel Ingots, etc. And so you have people undercutting on these stuff because they simply want it to sell. As long as I am selling for 30k, I am making a profit because I gather the materials myself.