Provided you have a specific event, you can use the event_list_attendees API to get all of the currently registered attendees.
What may work better is referenced here:
Pass Eventbrite attendee parameters to custom order confirmation page via redirect URL
When the order completes and the user is redirected, you can digest $event_id and $order_id to perform some action. All you'd need to do is put those in your redirect_url and then write your own logic to process those on your end.