You can generally sell tickets if they were purchased from AXS and if AXS Official Resale is available for the event. It's now available for most events at The O2 arena and OVO Arena Wembley.
If you are unable to sell your tickets this could be due to the promoter requesting the show is unavailable for resale, the ticket is VIP ticket or your ticket is priced as a special offer.
To check if you can sell your tickets, sign in to your account and click on the event under “Your Events.” If you see a “Sell Tickets” button, you can sell!