Trouble prepping Mac Pro 2008 to sell

I've been searching online for help with this for days but not finding much of use, hoping someone here can help me.

I have a mac pro tower from 2008 that I want to sell, I just got a new iMac in December.

I have a disk for Snow Leopard but I had upgraded through the app store to Mountain Lion.

I was not able to use Snow Leopard to do a new OSX install, it said I had to use Mountain Lion. This computer is not currently connected to the internet so I did not do a new Mountain Lion install. It seems from what I've read online that you can't actually just sell a computer with mountain lion on it because it's not a disk you bought but something downloaded and attached to your apple ID, if I understand that correctly.

I read somewhere I could erase the harddrive and then install from Snow Leopard.

I used disk utility to erase (the 7 passes) the hard drive, but not I am stuck with Disk Utility on startup - there's no way to even open the DVD drive at this point. It gives me options to go back into the erasing/formatting disk utility screen, connect to safari, use the time machine recovery, or install mountain lion, 2 of those requiring internet.

I can't figure out how I can install snow leopard from here. If I connect it to the internet and just re-install mountain lion, can I sell it that way? I'm unsure what to do from here, I just want to get the computer into a clean state where I can sell it.

Thanks for any help!