my macbook air won't let me download any apps off the app store, i just says "so and so failed to download, use the purchases page and try again" but it still doesn't download

1. Quit the App Store application.
2. Open Terminal.
3. Write " open $TMPDIR../C/ " In the command line and press enter. This should open a new finder window to private/var/folders/…/C
4. Find " com.apple.appstore " and move it to the trash.
5. Empty trash.
6. Open App Store, Install Application.
7. It should work perfectly, if it happens again repeat the process.

It seems my hypothesis was correct: Something about the App Store cache was weird, and forcing it to re-create solved the issue.