Some sellers will include their eBay ID on a printed invoice enclosed with the item.
You might also want to try backtracking. The item's location should be posted on the listing page. Look for tracking information that originates at or near the stated location in the listing.
Any idea why the tracking number is different than what's in your "My eBay"? Is/was your item being shipped from the US or UK via the Global Shipping Program, which uses an "overall" tracking number as well as displaying tracking for most of the individual carriers handling your item?
