StuffAlert searches the individual ebay Market (eBay US, UK etc.) you have selected. Believe it or not, the category lists are different as between the various ebay Markets. For example in US ebay you will find a Category "Cell Phones & PDAs" whilst in UK ebay the same category is called "Mobile & Home Phones". So although both markets speak English different words are used to say the same thing.
The good news is that StuffAlert mirrors the categories of the ebay site you are searching. If you do not select a category StuffAlert wil automatically select all categories to give you the best chance of finding the item you're looking for. Where ebay offers sub categories these will be offered to you so that you can make your search more specific.