A Shadow Weapon is a special weapon made in cooperation between the Ivory Tower and Black Anvil Guild following the signing of the Giran Treaty. The weapon magically projects its power onto another object and can only be used while this magical charge remains. Because of that, it can be purchased at a significantly lower price than the original weapon.
In the hands of a properly trained adventurer, however, it acts just like a real weapon. Its magic essentially uses the wielder's strength for its power, so unless the wielder has the appropriate level, the Shadow Weapon is worthless.
We don't sell the lowest level projectile weapons, so they became supplies for the Grand Masters; go ask them about it. The weapons sold in the shops have a higher level.
It looks to me that you don't have the necessary ability to wield one yet. Come back later after you've developed your skills some more.
(Projectile weapons can only be sold to level 40 or higher characters.)