i want to remove my old device apple i cloud to sell it

This explains what you need to do to totally erase it so that someone else can use it.

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Hi,

Thanks for asking this is not a big issue you have to sign out it from your old device if you are still unable to sign out then you can remove that device from your desktop by logged in. then you can sell it on your demands.