Do not ever accept a computer without the original install disks. Without install disks, you're pretty much "locked out" of doing anything with your computer other than what you already can do. Plus, if there's ever a problem that requires a reinstall, you're SOL.
If you do decide not to pursue the seller for the install disks, then I might suggest right after you buy that computer, run on down to the car dealership and make them sell you a car without keys.