I have the following Ebay API call that has successfully worked on Postman
https://svcs.sandbox.ebay.com/services/search/FindingService/v1?Operation-Name=findItemsByKeywords&Service-Version=1.0.0&Security-AppName=PetrosBe-Sells-SBX-9ac2147f3-b674cf2c&Response-Data-Format=JSON&REST-Payload&keywords=laptop