If you click on Reporting in Selling Manager, the next screen will probably say something along the lines of "generating report" etc because the default is set to current month, but if you bring up the dropdown menu for month (Period), you should be able to select to view sales records from previous months (mine goes back to Sepember 2012). At the top, it will also give you gross sale amount, Total cost (comprised of postage, eBay and PayPal fees, which is broken down at the end of the page), then net gain (gross sales - postage, eBay/PayPal fees).

The individual sales for that month are also itemised. 

I also have my weekly sales automatically downloaded to a CSV file, but you could probably set it to monthly so you would have a record on the computer, as well.