What I am asking about specifically is farmable items only that are not naturally sold by the guild NPC merchant. Example:
Here is a list of items sold by Cletae in the Lethercrafting Guild in Sandy:
http://wiki.ffxiclopedia.org/wiki/Cletae
Karakul Skin/Leather is not on that list. If I sell her either of those items (even though I can sell them to her because they are leathercrafting items) someone looking for those, can not buy them back right? What I am suggesting is that, since the Guild Merchants already have all their crafting items in their logs of what they can/can't buy, when somebody sells items not initially listed for ANY crafting item, open them up to sale.