One of the reasons why businesses like Google Adwords is that they are in control. Groupon and all others like them do not offer this capability.
I have been asking myself this question for over four years now.
What if?! If there were a web service where for example a restaurant could login and place its deal(s), the day(s) and hours the deal(s) is(are) visible and/or available, and where a user could do a quick search based upon his/her location to find what restaurants deals are available at that moment, would businesses and users still go to Groupon?