Just a thought....
I've seen it done in other games and I've always had a love/hate relationship with this, but:

When you go to sell a stack of Gridanian Walnuts to an NPC, the price depends on how many have sold recently?
Obviously each item would have a lower and upper cap.... but yea, if everyone is NPCing their walnuts, the vendors wont give you as much for them?
It could be how many walnuts have been sold to that particular NPC, in that region or just game wide (I've seen it done each way)

Also, while we are at it, what if certain NPCs paid more for certain kinds of things?
Like you'd get a few more gil for vendoring a ring at the GSM guild or a GSM merchant than you want a merchant who sells food

Im not for or against the idea... just thought it would be something different...