Hi @Anonymous User
Sounds to me that when you sold the NowTV Box at the Car Boot Sale you didn't factory reset the box to remove all your sensitive account details, hence the reason the person who bought the box is using your account where it looks like that they have bought a pass on it.
I would suggest speaking to live chat on what course of action needs to be done to rectify this.
Link here to live chat.
https://help.nowtv.com/contact-us/managing-my-devices
