All good questions... That you should have asked BEFORE listing the thing on eBay.
That being said, the short version is that there's no way to guarantee that someone cannot, under any circumstances, recover data from a HDD short of not selling the system with a HDD, or putting in a different HDD. You could run something like DBAN, which after a number of passes, would mean someone would have to have the resources of a government to be able to recover the data, but usually it takes about a couple of days of constant use to accomplish that.
There is also no way to delete everything except the OS, it just doesn't work that way. You can erase everything and reinstall the OS, but you can't reset the OS to factory conditions without erasing everything.
In this case you seem to have hoisted yourself by your own petard. These are all issues you should have worked out BEFORE listing and committing yourself to selling it on a specific schedule. At this point, assuming you actually have restore discs, the best thing you could probably do is just run to a Best Buy, Fry's, or whatever's in your area, and get a new HDD that's similar in spec to the one in the computer. Swap the old one for the new one, reinstall the OS, and sell it that way. It will be an added expense you'll have to eat, but it's a bargain compared to what someone could do with the data that may be stored on the drive now. You can also consider it penance for your impetus actions. Next time, you'll be unlikely to make the same mistake.
I am selling my Asus g73jh on ebay and their are two things i struggle to do.
The first is to permanently delete all my files so that not even god could access them. But people on the internet say that sometimes, your internet application stores passwords in a file so if that is true i would of course want to delete that and also any sneaky files of such that applications may add. Also my computer has facial recognition so i need o know basically how to delete everything except things like my OS and important things needed to run the computer.
The second is that i want to let the buyer open the computer, turn it on and have to go through all the first start up procedure so it can be like how i first started it up when it was brand new. Please help thanks.