Is the sandbox useless? Can we only trust the production API?
According to the docs GeteBayDetails should work without DetailName. That's how I had it working with a project I have just returned to. It now seems, contrary to the docs and in the sandbox at least, that is not the case. Even in the API Test Tool!!! This is not the first time I have had issues with the sandbox. Issues which have cost me days to establish that the error was not in-fact with me but an untrustworthy development environment. Pull your finger out eBay. This is just embarrassing.