When I test sandbox features I do the following. Lets say that I created my paypal sandbox account as firstname.lastname@example.org
which is the login I get on the front page of sandbox.
Then I create my business account and buyers account. By the looks of it you have entered your business sandbox account in opencart which is probably not right. Enter the email account you have to login to the frontpage of sandbox which in my case would have been email@example.com
and not firstname.lastname@example.org
To test the sand box feature on configured accounts I do the following.
1. Open a browser and login to the front page of sandbox.
2. Select the buyers account which you want to use later for open cart buying and click enter site.
3. Now leave these windows open, open a new tab in the browser window which you opened in step 1.
4. Log into opencart select your products and check out. This will give you the sandbox payment page.
5. Enter the password for the buyers account which you are logged in with (step 2).
And thats it. If you select sale I believe that this will result in a call back when the purchase has been made and result in an email being sent to the buyer from opencart after successful purchase. If you select authorize open cart will email already when you click checkout.
Please note that you will not receive any emails from paypal sandbox to your email account. These will only appear within the sandbox frontpage environment where you can view and delete emails.
Hope this helps.