Factory reset on a 2009 MBP I recently purchased a MBP it has El Capitan. The people who I purchased it from were unable to perform the factory reset. They tried for over an hour. So I have this really nice MBP that still has all of their information. I have all their log in information, so that we can reset it ourselves. They purchased it as a reconditioned unit 2 years ago they haven't really used it and have never made any updates.  It came with the box and all the installation disks. The disk is Snow Leopard. 
First and foremost what's the best way to process the reset?
Will I have to install Snow Leopard?
If so how do I install El Capitan​?
Is there a way to upgrade to Sierra? It's a mid 2009.
The best way to do a factory reset of a Mac is basically to wipe the hard drive and reinstall from scratch. You can boot off the installation DVD (insert the DVD, reboot and hold down the ALT key to get the boot selection menu), then once you see the first page of the install wizard choose Disk Utility from the menubar to wipe the drive. Then you can continue the install to get a fresh copy of Snow Leopard. Once that is complete you'll be able to freely update to El Capitan from the Mac App Store.

Unfortunately it looks like you won't be able to upgrade to Sierra. The earliest MacBook Pro supported by Sierra is the Mid 2010 model. https://www.apple.com/macos/how-to-upgrade/#hardware-requirements

However if this MacBook hasn't been used since reconditioning, then perhaps just creating a new account for yourself and deleting their account would be enough. Just select System Preferences from the Apple menu, and click on "Accounts". You'll be able to create a new account there (make sure to tick the "Allow user to administer this computer" box), and then once you've logged into your new account you can use the same Accounts pane to delete their account.