How can I reset the firmware password on a MacBook Air? I purchased a MacBook Air from someone 4 months ago and everything was working perfectly. There is no user name or password needed to log in and I've never had problems using it.

Now I would like to reinstall OS X but I can't do it because it's asking for a firmware password when booting with Option ⌥ held down. It's 4 months now and I've lost the seller's phone number to ask for password.

I've tried to reset NVRM but those keys are disabled. I can't boot from external HD when I turn on the system. What is the reset procedure?
To reset the firmware password it looks like you have to take it in for service at your  nearest Apple Support location as noted in the following Apple Support Article MacBook Air (Late 2010) and later, MacBook Pro (Early 2011) and later, iMac (Mid 2011) and later, Mac mini (Mid 2011): Recovering a lost firmware password

Symptoms
Learn what to do if you no longer remember the firmware password that you've set or if you see a password dialog when you try to select a startup volume on a MacBook Air (Late 2010) and later, MacBook Pro (Early 2011) and later, iMac (Mid 2011) and later, or Mac mini (Mid 2011) computer.
Resolution
Only Apple retail stores or Apple Authorized Service Providers can unlock these computers protected by a firmware password.
If you cannot remember the firmware password for your Mac, schedule a service appointment with either an Apple Retail Store or an Apple Authorized Service Provider.  If you plan to visit an Apple Retail store, please make a reservation at the Genius Bar using http://www.apple.com/retail/geniusbar/ (available in some countries only).

As a side note you will need the original proof of purchase.