If you go to Seller Admin > Transaction Log, there's a column that shows any Discount(s) applied to each sale. You could pick the date range and/or specific product you're interested in, click View Log to see just those sales, then you could also click Download Log to save that log view to a file on your computer. Once you unzip and open that file in a spreadsheet program like Excel, iWork Number, or (free) Gnumeric, you can sort rows according to the Discount column, which should group sales with similar discounts together.
