If the item is on your Inventory page, you can find the item ID by selecting the item on the page. 

Click the More button under the Actions header. Click Edit. 

You will find the item ID at the top of the picture. 

You can find the item ID if the item is listed by going to the Customer View for the listing.  On your Listings page, under the Manual tab, find the item and under the Tools header, click the silhouette icon, or Customer View. 

You can find the item number above the claim info for your item. 

Or scroll down and the item ID is the number in the claim link listed above the QR code.