Buy orders have been discussed before at length a few years ago found here: Fixing WoW Economy: Buy Orders - Community / General Discussion - World of Warcraft Forums (blizzard.com) Not my thread it was recommended as similar when I went to post this thread.
My suggestion on this topic using aforementioned thread would be to use the crafting order system that we now have since this time which essentially allows for a buy orders of specifically crafted items.
Essentially add breakouts for the gathering professions in this process. Then allow form full and partial fill completion, of crafting reagents. Actually the “Place Crafting Order” menu already has it for Trade Goods, just need to add the raw resources.