mac doesn't re-sell returns. ever. even if you buy an item, return it 5 minutes later to the same MA, they still have to throw them out for sanitary reasons.
shadesticks are notorious for breaking in the way you described. i have all except 2 of them, and i've had problems with like 4 of them, nordstrom has always exchanged them for me, no problem. however, the method listed above--take off the plastic "cap" at the end, stick a pencil (eraser end) up and push, and "click" the shadestick back into place--really works well. i've used it several times and its really easy and fixes the shadestick..