Here is what the previous owner should have done when giving up the laptop:
https://support.apple.com/en-us/HT201065
If you have the Snow Leopard install DVD, then boot it and launch Disk Utility. Select the physical drive in the left pane. I find it easiest to the select the "Partition" tab in the right pane and select "1 Partition" from the drop down list. Make sure "MacOS Extended (Journaled)" is selected. You can also give it a name. Disk Utility's default is usually "Untitled". The macOS default for a boot volume is "Macintosh HD". Double-check the partitioning scheme is listed as "GUID" in Disk Utility. Once the disk is properly erased, then you can select the "Reinstall OSX" option. Make sure to completely update Snow Leopard with all the software updates before upgrading to macOS 10.11.
Edit: Once you have Snow Leopard installed, then you can upgrade macOS to 10.11 El Capitan. If you have access to another Mac, then you can download and create a bootable macOS 10.11 USB installer to go directly to macOS 10.11 instead of dealing with 10.6. If you have problems with 10.11 let me know. If you bypass the clean install of 10.6, then make sure to properly erase the drive as I described before installing 10.11.