Currently Being ModeratedJan 25, 2013 4:45 PM (in response to Bylogan)
The first thing you want to do after buying a used computer is to erase the boot volume and reinstall the original software. If it shipped with OS X 10.7 or later, you do that using Internet Recovery. If it shipped with an older version of OS X, you need the installation media that came with it.
Currently Being ModeratedJan 26, 2013 9:41 AM (in response to Bylogan)
Boot from the USB key and launch Disk Utility from the Utilities menu. Erase the boot volume (usually called "Macintosh HD," though it may have a different name.) Quit Disk Utility and install OS X. Go through the initial setup process, during which you'll have the opportunity to transfer data from another Mac or PC, if you want to. Then run Software Update.