if you create a facebook event from a business site you can add an URL where visitors are redirected to an external shop system where they can buy presale tickets.
But i see that there is a integration possible, where you can show, select and buy presale tickets directly in facebook. Sometimes you can just select tickets and when you press "buy" you getting redirected to an external shop-system with your preselected-tickets and in this example you can also pay in fb.
How is that integrated?
The page under the redirect link contains no additional informations, just an fb-app-id, but i found no api for this features. Can´t believe fb makes an secret special api just for one ticket-service.