Cannot Install or Update Apps in App Store - Error 13
I seem to have fixed the problem.

I used opensnoop to follow all file accesses while the app store was running. 
I spotted that
~/Library/Cookies/com.apple.appstore.plist
was being accessed - this file had not been deleted by Safari when I cleared all cookies!
The file had incorrect permissions - I deleted it.
Now the App Store seems to be working again.

Opensnoop is a great tool!