You know a buyer isn't really interested in buying a car (yet, or the car they are looking at) if they're a time-waster.
Examples: "Well I don't get paid until Friday" or "Can you hold it for me so I can make sure the wife is OK with it?"

You know a seller isn't really interested in selling a car (yet) if they're a time-waster.
Examples: No price listed, no pictures available

They aren't looking to sell this car - at least not seriously, yet. If they were, it'd have a dollar amount. Even if it's absurd. It needs to start somewhere.