This is kind of off topic, but i just realized that i have been looking on the "joined" stamps rather than the posted "stamps" on posts, and it has made me tottaly confused.
Anyways back to topic. As dislekcia said, it is in your shop pool now, this happens with all quest items, once you unlock them. So if your guild locker is full, don't worry about not adding it. (Hint/Spoiler : In the cursed oasis, one of the bosses will drop an item that you can put in your locker before you acctualy unlock it to your shop pool)