You will have to do the Internet Recovery, it will download the OS (10.7) and install it as if you haven't used it before.
I believe that the Internet Recovery is press Option and the R at the same as soon as you hear the boot up chime. I could be wrong on this, it's been a while since I did a Internet Recovery. :/
NOTE: This will install 10.7 and not 10.8 theres no legal way (although you can install 10.8 on the machine) to install 10.8 and sell the machine. It's got to do with the licensing.
Hugh