You can obtain previous buyer emails from your Admin > View/Download Transaction Log. Note the log display defaults to show only the past month, but you can adjust the displayed date range at the top of that page. If you have a lot of emails to grab out of there, you can download the log to a file that you can open in a spreadsheet program like Excel or Gnumeric, so you can select the column of emails to copy and paste into, say, Notepad to produce a plain text list of emails.