1 thing to fix this massive undercutting nonsense.

Make it so you can never sell it on the marketboard for less than what you'd vender it for. This stops certain high level things (such as Clear Demimateria III) for being sold for less than their actual worth...like how Guild Wars 2 does it. There, we've fixed a lot of the nonsense for things, such as Clear Demimateria III being sold for less than 5K gold (it can be vender'd for that much people).