It's not fair to restrict the items that way. So crafters need the item to craft - why do they need to craft? To make some Gil right? So when someone says I need that for crafting they are basically saying I need that for gil. Everyone needs gil. If I sell the item, a crafter buys it and sells the crafted item for a profit (otherwise they should just sell the raw mats instead). Do you see how it's greedy to want 100% of the profit?

It doesn't make sense anyway since why shouldn't an adventurer be able to gather treasure / materials to sell? The difference with locked gear is that it's untradeable - making crafting mats untradeable would force crafters to farm dungeons which they may not like, and would make the final crafted products much more expensive (less crafters would have access to mats so the supply would be very low in comparison to demand)!