Once you have purchased an e-ticket they cannot be sold again as the e-ticket will be addressed to the person who made the purchase. As in order to gain access to the event tickets will be cross-checked and the identification on the e-ticket and the attendee should be the same or you will be refused entry. Once a e-ticket has been purchased you cannot cancel as they are non-refundable, unless the event is cancelled (see refund policy).
