Yes, it's possible to delete the messages without any chance of restoration. In fact, the option available within your operating system already does that. Simply go to Settings -> General -> Reset -> Erase All Content & Settings and that's it. Make a complete reset and everything will be deleted without any chance of recovery. Secondly, delete any backup you have on your phone or computer that is related to the content available on the iPhone you want to sell.
Additionally, use iFunBox and go to Raw File System -> Device -> Var -> Mobile ->Library -> SMS. Delete all the three databases and the messages will be wiped from the device.
