If an item is lost under the GSP after it has left the hub then sellers are not responsible.
The buyer opens a not received dispute and you make sure ebay know the tracking says Delivered to the hub
You have to make sure ebay knew it was gsp and have tracking showing Delivered to the hub
You have to make sure it is on the item in your sold section and on any not received dispute.
If the item was lost or damaged after leaving the hub then ebay refunds the buyer and you keep the payment
https://www.ebay.co.uk/help/global-shipping-programme/default/global-shipping-programme?id=4646
Did you refund yourself?
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