Hm, well our Updates and Newsletters feature can only send messages to ALL previous buyers of a given product; there's no way to sort by last-purchase date.
You could go into your Seller Admin > View/Download Transaction Log, set the date range to show only transactions from 6 months ago and older, maybe select sales of only a specific product if you wish, then View Log and Download Log to save that to your computer. Once you unzip the downloaded file, you can open it in a spreadsheet program like Excel or (free) Gnumeric to, say, select and copy the column of email addresses to paste into a text file.
Then you could submit those addresses to a third-party email-marketing specialist service like Aweber to send an email blast to all those addresses containing a Cart Discount or Item Discount code you'd created that you're revealing only to them.
