Go to Seller Admin > View/Download Transaction Log, change the date range (defaults to show only the past month, but you can go all the way back to 2006 if you want) and View Log, then Download Log. Once you unzip the downloaded file, you can open the log in a spreadsheet program like Excel, or free alternatives like OpenOffice or Gnumeric, and sort by the Payer Email column (for frequent shoppers) or Amount (for big spenders).