I had to do something similar a few years ago with an early 2080 model, circa mid 1980's. I was put onto the
Telescope Blue Book and worked out the scope was probably worth a couple of hundred at most. Sadly those scopes haven't really kept their value, so don't pay too much. And sadly the Telescope Blue Book hasn't been updated since 2007! You should also have on hand what a new replacement similar model would cost these days and keep that in mind. You might find they paid a mint for their model back in the day, but the equivalent today is a lot cheaper.