If it's fairly new use the recovery partition. Boot into it, and erase the disk (suggest some sort of secure erase) using disk utility. Then use internet recovery to load OS X.
Other than that, especially if it's a pre-2011 Mac (or 2010 13") you are going to need either a boot disc, bootable USB drive, or another Mac via Firewire to erase the disc. But now you have no way to install OS X.
The Apple Store sells the OS X Lion USB Thumb Drive for $69, which will both allow you to securely erase your drive as well as load OS X onto it. It is bootable and is what I suggest you get to do what you need. The buyer is going to need a way to restore the OS at some point anyway. iLife will not be included however.