Quote from AnUnknownMiner
This is what is causing it
Caused by: java.lang.RuntimeException: Language key 'mainCommand' not found in the language.conf
Which probably means that whatever you did wasn't recognized by the plugin. Maybe try re-installing it or see if their is a new version.
He deleted the language.conf file....... or it is messed up.
Basically delete language.conf and just reload/restart the server.
If it still doesn't work update the plugin. Buycraft works fine for me.