Every eBay listing, whether its an auction or Buy it Now sale, has an item number to identify it. While this number is primarily used to refer to a listing when making a complaint or when sending a payment outside an automated system such as PayPal, the advanced search tool also uses item numbers to find specific listings.
Item Numbers on Listing Pages
Every listing on eBay displays its item number in the Description tab. Scroll down below the "People who viewed this item also viewed" section and look on the right side of the page, directly below the Report Item link. If you're having trouble finding the number, press Ctrl-F and search the page for eBay item number.
Item Numbers in My eBay
To find the item number for a product you're bidding on or selling, look directly below the item's name on the Summary page of My eBay. To see the item number for older items you've bought or sold, click Purchase History or Sold, respectively.