What you're talking about is an inherent trait of a free market. Change that or control it and it's not a free self-regulated market anymore. What Tyrone said is right, no one would list anything on the GTL at a price cheaper than what an NPC would pay for it. Sellers can dictate what they sell stuff at and despite big price dips being annoying, with popular products, they're always temporary and value will restore itself, while whoever undercuts by a lot instead of $1 will lose out on a lot of profit.
Fun fact: no it's not. You can sell berries to NPCs. Not like I'd be against removing that feature either tbh