Follow the instructions above and I got it fixed. No idea why it happened in the first place.

To save you sometimes, just copy and paste those lines below in Terminal. Logout, login back and try purchasing an app again.

rm ~/Library/Caches/com.apple.appstore/Cache.db;
rm ~/Library/Preferences/com.apple.appstore.plist;
rm ~/Library/Preferences/com.apple.storeagent.plist;
rm ~/Library/Cookies/com.apple.appstore.plist