Mac – Appstore Policy: Why do apps stop using appstore and instead install from web sites

The Reset cache solved my problem in the iTunes advanced preferences:

Since then I can install and upgrade applications from AppStore.

I had a similar problem with one of the Xcode 4 versions. What worked for me was to log in to Apple Developer Network (this will work with your free developer account login), at https://developer.apple.com/xcode/

Although that page says to get the most recent version of Xcode at the App store, there is a link that says "For past versions of Xcode or additional tools -- view downloads".

• Click the downloads link
• Unselect all of the checkboxes on the left except "Developer Tools", type "xcode" in the box at the top of the left column, and hit Enter
• The top result will be Xcode 5 - click the disclosure triangle to view the download link for it.
• Download the .dmg, double-click on it to mount it and run the installer

Note that the downloads page says that Xcode 5 requires OS X 10.8.4.

As I mentioned, this worked for me when the App Store locked me out with one of the Xcode 4 versions. If this happened to you, please leave a comment indicating whether this solution works for you.