NB: When 2Checkout is running test mode, the mode is clearly identified on the 2Checkout Web Page in big red letter at the top of the page.
Your 2Checkout should be configured to run with the 'demo' parameter, which will allow Secure-eBook to control whether you are running in test or production mode.
If your 2Checkout account is configured to force production or test mode, please make sure that your Secure-eBook 2CheckOut configuration is set to match.
When changing between 'test' and 'production mode', you must clear your browser's cookies, as 2CheckOut keeps some information that might prevent the test or production transaction to operate appropriately.
You can find more information about the demo mode on the 2CheckOut Web site: http://www.2checkout.com/community/blog/newest/demo-parameter-changes/