Online booking engines can be easily customized to handle over-booking. The issue I'm looking at is that online booking engines don't have accurate (and real-time) supply picture today. If this was available, it would become possible to make reservations without the need to get confirmations from the provider.

Take sites like froomz.com or venuecricket.com, for instance. They are online marketplaces for renting spaces. They have many small-time providers renting out there dance studios or workshops. If such providers exclusively use the booking engines provided by the froomz/venuecricket, there are no issues. But if they also use Outlook to handle offline reservations, then keeping it in sync with online booking engines becomes a big challenge. You might end up booking venues only to receive a failed reservation several hours later resulting in extremely unhappy customers who will likely write bad reviews about the site and the venue.