Thank you for contacting Steam Support.
Are you attempting to make the purchase through Steam or with a web browser using the storefront webpage?
If using Steam, try the following:
If using a PC
-Exit Steam, then go to the location that Steam is installed to. By default this is C:\Program Files\Steam directory (this is the default with a Steam installation - if you set a different installation directory, you will need to browse to it).
-Delete all of the files except for the following:
- \SteamApps\ and \Userdata\
If using a Mac:
-Exit Steam and go to the folder Users/[username]/Library/Application Support/Steam
-Delete all of the files in this folder except for SteamApps and Userdata.
For all operating systems:
-Ensure that you are not using any IP proxy or VPN software. These applications must be completely deactivated when purchasing through Steam.
Restart your computer and test the issue again.
Note: This process will not affect your currently installed games.
If the issue still persists, try using a web browser (Internet Explorer 7 or higher on PC, or Safari if on a Mac) to make the purchase using The Steam Autumn Sale 2013
If it still fails, try logging into your Steam account and making the purchase directly through the Store interface.
Please let us know if you have further questions or concerns.