Erased HDD, cannot install OSX

I recently erased my HDD ready to sell my MacBook Air 2008 and now cannot install an OS onto the machine as I bought it second hand and it is asking for a password used to purchase Lion. Is there anything I can do to install an OS onto this laptop? It is not imperative as the machine is being sold for parts but I'd like to know how to do this in future! This is rather annoying as I feel that I could have avoided this somehow, please help!

Did I make a mistake by erasing the hard drive? Is there another way in restoring macbook to factory settings?