Error 13.... Follow these steps exactly to the letter...
Note: This is the updated Mavericks version of the error 13 fix.. Do not use it if running Mountain Lion or earlier... It may not work correctly!
Back up all data.
Triple-click anywhere in the line below on this page to select it:
~/Library/Cookies/com.apple.appstore.cookies
Right-click or control-click the line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item selected. Quit the application if it's running. Move the selected item to the Trash. Relaunch the application and test.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.
(Thanks to Linc Davis for this solution.. I cannot find his original thread but I had kept a local copy of this 'fix')