Just bougt a movie on Ebay, "The Castle", When starting the disk I get"Cannot Play" never had this message, any help appreciated.
Was it an international purchase? The dvd is probably incompatible with your player because it is designed for another region. Try playing it on a computer. Check this link out for more details http://www.ebay.com/gds/DVD-REGION-CODES-EXPLAINED-What-is-a-REGION-CODE-/10000000004641793/g.html
One way to find out if it's a region code issue is to download AnyDVD and then try playing it in your computer player. Note that buying foreign movie discs can also result in a different aspect ratio as well. If it turns out that AnyDVD allows you to play the disc on your computer, then rip it to your hard drive using an application such as DVD Shrink and then burn it to a blank media.