if you cant use the hwardware method, then go with 35 passes and leave it there till it finishes..might take a few days. if the drive is fine after that, then thats quite the stability test as wellThat made me lol. Seriously though you cant depend on the buyer to not snoop through your data and erase it. One hdd i bought was a refurbed drive fresh from the factory so there was no issue there, but a smaller 40gb portable drive i got wasnt and i didnt even think of trying to snoop, just loaded my own stuff on. Not everyone is that honest though, so if you do happen to have videos of you and the wife and they get leaked out in a situation like this its your own fault.
Personally i use wipe drive to zero drives, maybe a second or third pass just to feel more secure even if it makes no difference.
the only thing is make sure its properly cooled, cooking for 2-3 days can damage a good drive.