Well, moving is always good. But moving isn’t exercise. If someone is looking for a fitness machine, I presume they are looking to get exercise.
Sitting on a couch and moving your legs is kind of like walking to the mailbox and back a few times. It’s better than sitting on the couch, but it wouldn’t actually be exercise.
A used spin bike would be a much more appropriate purchase for actual exercise - even if you don’t usually ride hard, it has the potential to actually allow you to get in a moderate workout from time to time.