Tomorrowland runs an official ticket exchange, where buyers can resell their tickets, through their website. The website lets users safely buy and resell their tickets. At the moment however it is closed.
Be very careful when buying your ticket second-hand. Ticket fraud on the internet is a very real thing, and if you fall victim to it there is not much you can do.
We try and help to the best of our ability, but when in doubt consult the official Tomorrowland website.
When you buy your ticket you will receive a bracelet that you can activate online by registering your name. We advise festival-goers to wait as long as possible to activate their bracelet, in case something happens and cannot go. As long as the bracelet is not activated, and their is no name attached to your ticket, then it can be re-sold.
Once it is activated it cannot be resold because it will be registered under the users name and ID. You will need to bring your passport when entering Tomorrowland to match your bracelet with the registered person.
If, however, the bracelet is not activated, you can buy it and then register it in a special tent on location, after waiting in an extremely long queue (so once you are sure you are going we do recommend registering because it sucks to stand in line for hours in the sun).
This seems to be a popular service for buying Tomorrowland tickets. Although tickets go for about two or three times the original price, they are unregistered, and should be fine. Viagogo says that it guarantees the tickets and it seems like they are legitimate, although some customers have reported problems trying to get in touch with them, or receiving their tickets late.