The caps probably leaked while in storage. Just because it hasn't been used doesn't mean it is free of bad caps--the caps are as old as those on an LC that was used in an elementary school for 13 years.

Also, sometimes sellers think that a part is new/working just because it's in an Apple Service Source box when it really could be something older/used. I got a dead Classic analog board on eBay once that was advertised as new. However, it was actually an old dead board that someone had simply packaged inside an Apple box. The dirt on the power switch was evidence that it was used--a new part wouldn't have the grime that comes from a finger flipping the rocker every day.