In the Market place, when a 'Level 1' (me), attempts to buy a Wooden Round Shield, it tells me that I need to be a 'Level 3' or higher to buy and or use the item. But, when I go to the Artifact Store, it allows me to buy the item, and use it ?
The thing is that players below level 3 are not allowed to use the market.
So that's why it's not allowing you to buy the shield there...
At you level you can also buy a Tower Shield if you have enough money and it is in stock. But you just can't use it.