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You can import the eBay Trading API Sandbox SSL server cert as follows:
1. Access the Trading API gateway Sandbox URL https://api.sandbox.ebay.com from firefox browser
2. When the server uerl is rendered, right click the page -> select View Page Info -> click Security -> click View Certificate -> click Details -> click Export as shown below :
NOTE. Production SSL cert version can be different compared to Sandbox one. Please make sure to get the Production SSL root CA cert from https://api.ebay.com in order to make Trading API call against production environment.