Have you tried clearing the cache in DNN and rebooting the server? To clear the cache in DNN login as "host" and go to the Host Settings page in the Host Menu. Click the “Restart Application" link at the bottom of the host settings page, if you need more help see this article.
Also:
1. How do you know that the test transactions are coming from the cart
2. Do you have another site or instance of the buynow module on your site that is configured in test mode?
I would also recommend upgrading to the latest version (3.99).
Christian