EAN value for books. (This is used more commonly in European countries.) If you know a book's EAN, you can use this instead of the eBay CatalogItem or Reference ID to search for that book.
Max length of corresponding value: 13
ISBN
ISBN-10 or ISBN-13 value for books. (The string length of ProductID indicates whether the ID is 10 or 13 characters.) If you know a book's ISBN, you can use this instead of the eBay CatalogItem or Reference ID to search for that book.
Max length of corresponding value: 13
Reference
The global reference ID for an eBay catalog product. A reference ID is a fixed reference to a product (regardless of version). Use this instead of CatalogItem when you know a product's reference ID (because older CatalogItem IDs are more likely to become obsolete over time.) One reference ID can be associated with multiple CatalogItem IDs. To determine valid reference IDs for products, first call FindProducts with QueryKeywords (or ProductID). Each product in the response includes a reference ID.
UPC
UPC value for products in Music (e.g., CDs), DVDs & Movies, and Video Games categories (or domains). If you know a product's UPC, you can use this instead of the eBay CatalogItem or Reference ID to search for that product.
