It's the unfortunate fate of farming not being viewed as productive enough, really. When the endgame items one may be eyeballing cost tens, if not hundreds of millions (for HQs), not many people are going to want to be wasting their time and clogging their AH slots for sub-100k profits. Seeing onions brought up, stacks on Sylph are only going for 6k while Asura is showing 180-200k. Super big difference, obviously, but something like this also shouldn't be considered the norm when it comes to supply and demand.

Overall, the game has had some issues when it comes to material rarity at various points, and it's something that has made crafting something questionable to pursue for many. Why craft NQs for a loss when you can sell the ingredients for that much more? Such also contributes to the absurd gap in HQ prices, further factoring in things like tiering or the actual differences from NQ (some justified, some not so much). The crafting shields are serving as something the game has arguably lacked for its life time, and that's material sinks. GP items flirted with the idea, but not really to the degree they'd have a major impact on the economy. And if costs were too obscene, sometimes players just skipped those days altogether.

So, how do we fix the supply issue? Well, vendor mats are one angle, particularly with conquest. These also double as a gil sink, which may or may not be needed depending on who you ask. I can't say I'm wild about anything having an exclusive source, which means bottlenecking things to bosses/events is spotty. This leaves HELMing, chocobo digging, gardening, and monster slaying as alternative venues. I'd include chest/coffers, but they kinda stopped being used post-RotZ. Nonetheless, chests in Giddeus dropping a dozen onions wouldn't throw of the natural order, I'd say, so maybe such things do warrant a buff. Otherwise, since player demand on many things is virtually non-existent, I'd say more NPC quests are in order that pay out substantial gil or other perks. It might not be the ideal player-to-player economy, but it'd also be better than what we have going for us now.