When you list an item in eBay Sandbox with a PayPal Email Address, ensure that you use an address that is registered and confirmed in PayPal Sandbox. If both these conditions are not met, PayPal will not complete the required processing and the item will remain unpaid.
If the PayPal email address that you used to list the item is not registered or confirmed, the amount that you paid will remain unclaimed:
If the amount is unclaimed, it will not be processed and the item will not be marked as paid.
To ensure that the payment is process:
Log into your PayPal developer account and ensure that the email address you wish you use to list the item with is registered:
Check if the email address for this account is confirmed. If it is not, confirm it: