I've noticed that most of the car sellers here on NL lists 'factory fitted ac' as one of the many available options on the vehicle they intend to sell?

It makes no sense whatsoever, unless non-factory ac's do exist in vehicles that I don't know about. 

Or are you trying to distinguish from the non-factory ac in some danfo's, the dashboard fan? Please enlighten me

Some cars that don't come with ACs from the factory. Recently I came across an Chevy Aveo (SV Model) without factory AC, the owner was looking into how to manually have one fitted in. Some people also buy cars in the absolute base model, which means very little car features comes with it. From what I hear though, non-factory fitted AC cars are very rare.