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As a buyer, how can I find out the shipment tracking information?
Currently, if the seller ships the item via USPS or UPS, they can enter the shipment tracking number for the item and you can see it from MyeBay -> Won list, locate the item, click on the Total Price for the item and it will take you to the View Order Details page.
Alternately, when you locate the item in the Won list, you can click on the title and it will take you to the View Item page. Click on the View Order Details page.
Here is a sample of the View Order Details page:
You can also use the transaction API calls such as GetItemTransactions and get the information from the ShipmentTrackingNumber field.