How about an adventure request board? someone posts what synth materials they want and a reward for that item. i know we already have the bazaar request thing but that only works if you have at least one of the item you are looking for and someone happens to see your character/retainer.

I'm talking about a single point in every town (probably the adventures guild) where everyone can see what items people are looking (without needing to already have one) or can search by item to see if someone is looking for an item they already have. people can then adventure and hunt for the items that will reward them quickly and give more than selling to npc

for the person who is seeking the material they would look through a list of materials of the same category and select the specific one they want. then are asked how many they are looking for and how much they will pay for each quality (nq to +3)