Reformat it using the Mac OS X Install CDs. You'll lose everything on the drive, but at least it'll be a computer with your user accounts and data on it.
If you don't have the Mac OS X Install CDs, you'll need to purchase them. The seller should have included the CDs/DVDs with the computer -- there is no legal use for them other than the computer they came with, so if he/she didn't include them, you may want to contact them and have them send the CDs/DVDs to you.