Trying to Erase Drive

Hi,

I have a Macbook model A1181 running on version 10.5.8 that I would like to sell. I need to erase the hard drive but when I go through the disk utility option, the erase function is disabled. When I restart the computer while pressing the command+R buttons, nothing happens, except the utilities folder is opened upon start up. I've also tried erasing through the terminal but the error message I received was "Could not unmount disk for zeroing. Partitioning encountered error on disk disk0: Could not unmount disk (-10000)." If anyone is familiar with this error message or these issues, PLEASE HELP!