From ebay documentation belowl link you are limited to few categories only!! http://developer.ebay.com/DevZone/finding/CallRef/findItemsByProduct.html#findItemsByProduct
ISBN The ISBN-10 or ISBN-13 value for books. If you know a book's ISBN, you can use this instead of the eBay Reference ID to search for that book.
UPC The UPC value for products in Music (e.g., CDs), DVD and Movie, and Video Game categories (or domains). If you know a product's UPC number, you can use this value instead of the eBay Reference ID to search for that product.
EAN The EAN value for books (most commonly used in European countries). If you know a book's EAN, you can use this instead of the eBay Reference ID to search for that book.
As an example, try this sample upc = 883929106646 (Its a upccode for a movie)
http://www.ebay.com/sch/i.html?_nkw=883929106646 Here search ebay.com with valid UPC (Music category). Result : Positive
http://www.upcdatabase.com/item/883929106646 UPC database has a positive hit
positive xml response [Don't forget to change MyAppID above]
Conclusion: Nothing wrong with your query. Just ebay doesnt have upc/ean from all categories. EBAY Limitation