Please submit a contact-us request for "I need to link my eBay and PayPal Sandbox accounts." and we will happy to assist you.
Please navigate to URL https://developer.ebay.com/contact-us and follow the steps given below: 1. Go to the "Help with the eBay Sandbox" section. 2. Open the "I need to link my eBay and PayPal Sandbox accounts" 3. Fill the mentioned information and Submit a request.