when i try to buy mountain lion it says app store error (100) try later what should i do

Try here first > Mac App Store: "An unknown error occurred (100)" when purchasing

If that doesn't help, try the following:

Disable anti virus software.

Turn off the Firewall in System Preferences > Security (or Security & Privacy) > Firewall

If that doesn't help, open the Finder. From the Finder menu bar click Go > Go to Folder

Type this exactly as you see it here:

~/Library/Caches/com.apple.appstore/Cache.db

Click Go

Move the Cache.db file to the Trash.

Now here:

~/Library/Preferences

Click Go

Move these files from the Preferences folder to the Trash.

com.apple.appstore.plist

com.apple.storeagent.pllist

Now here:

~/Library/Cookies

Click Go

Move the com.apple.appstore.plist file from the Cookies folder to the Trash.

Empty the Trash, try the App Store.