They are not set in the router as such. However the router serial no is 'registered' to your account. When it's factory reset. it will connect to the PlusNet servers and automatically reconfigure with your username/password.
If you want to sell you need to ask for it to be deregistered from your account. You could try the online chat or maybe one of the PN help team will spot this and do it for you.
TBH I'd be inclined to hang on to it as a spare if I were you, unless of course you already have another spare. It's also worth having a PN supplied router to use if you have problems and need support they will be able to help much easier with their supplied router connected.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
