What's the logic behind the changing availability of movies on iTunes? For example, Harry Potter and the Deathly Hallows Parts 1 and 2 used to be available for purchase, but now they aren't.

It's a licensing issue. Apple (and other entities) are allowed to sell such and such a hunk of media until the media's owner says "Stop!" For example, the store once carried Frank Zappa's music, but no longer does because the Zappa estate revoked the license. In this case, someone involved with these Harry Potter movies issued the order to remove the movies or the movies had a limited-release license from the get-go. This often happens when there's a new version of a movie being issued--a director's cut, for instance. By making the "old" version unavailable, the movie company hopes that you'll be forced to purchase an enhanced (sometimes more expensive) version.

The limits of emailing videos

Finally, reader Phyllis Towns writes:

I take a lot of videos of grandchildren and pets with my iPhone that I would like to email to family members, but so far have not been able to do so. Is there a way to do this?