I used the Java SDK from https://github.com/eBay/event-notification-java-sdk and was able to successfully save my endpoint in "Alerts & Notifications".
When clicking on "Send test notification", I receive this error message:Notification delivery failed with HTTP status code 204 from https://ebapi.<myurl>.de/webhook. Please ensure that the marketplace account deletion notification endpoint is ready to receive notifications.
The documentation states that 204 is an acceptable response:
The callback URL should immediately acknowledge each eBay marketplace account deletion/closure notification with an HTTP status code indicating a successful response.
202 Accepted, and
204 No Content are all acceptable.
Also, I would have expected the SDK to send a correct status code. :)