1) In certain instances at the beginning that shield can be very useful. I won't spoil you with where unless you really want me to. Just make sure that you equip your front character with it, as most enemy arrow fire will be directed at that person. As you get further into the game and your armor class gets better, consider replacing the shield with one that gives you better all around protection. Alternatively, if you have a multi-/dual classed mage who can use it, consider giving him the shield to protect against spells being interrupted by missile fire.
2) That's how the merchants make a profit - buy low and sell high
