It's difficult to say without seeing it.
If it's at the bank account stage, it suggests a problem with the details you entered. I tried new details for my son, just to see if there was a general error on the site in that part of the transaction currently.
The sort code doesn't need hyphens for...
I've just listed my son's seat for tomorrow night and it went through fine first time.
The sell-side of the exchange not being open, does not stop the sell-side from operating when the club open it.