If you contribute paid content to the marketplace, you can view reports on that content's performance. Some of the things you can see are what was purchased, when, how much for, coupons used and much more.
To be able to see this information, the Payments reporting tab must be enabled:
Go to Portal Settings->Configuration.
Scroll down to 'Payment' and ensure that 'Content Provider Dashboard' is ticked.
Save at the very bottom of this screen, and you're done!
You may now go to Administer and you should see an extra tab - 'Payments' - where you can see reports regarding purchases of your content.