How Do You Know if Movie is Public Domain?

[hershon]

I'm interested in trying to sell cult movies I've collected on Ebay but I'm not exactly sure what constitutes if a movie is in the public domain and how do you find out if it is or isn't?
If a movie is over 50 years old and has not been released on DVD or VHS, does that make it in the public domain. A few years ago I was selling a video of W.C. Fields "Man on the Flying Trapeze" on Ebay and I eventually got a form cease and desist email from Turner Classic Films & ended my listings of it. Can anyone clarify this for me. Thanks

[eisforelectronic]

I would think very few movies if any would be public domain.