Did you ask the manager of the shop to check their CCTV? You can complain to the police if you can get an image of him on CCTV. After all a convenience store is quite small and there are never too many people in it. So you may even remember what he looked like if he was the only customer other than you at that time. For all you know he may be a repeated offender and the police may already be looking for him in connection with other cases. Such people should not be allowed to get away. Whenever such an incident happens you just end up constantly watching your back for the rest of your life. It destroys your peace of mind more than anything else.