If you equip a Mercenary with a Sharp Sword then change their job to Archer or probably Guardian you lose the Sharp Sword. It still shows up in the # purchased count in the store, but you can't equip it on anybody, even if you convert your Archer/Guardian back into a Mercenary. It's simply lost to the winds. If you buy a second Sharp Sword you can equip that normally, but the first is still gone.