Can't Restore iMac to Factory Settings

Sorry for the lengthy post - i'm trying to provide as much detail as possible.

I'm trying to reset a Mid-2010 iMac (iMac11,3 i3) to factory settings to sell it. It's running OSX 10.6.8 Snow Leopard. I'm running into issues every option I try-

1. I can't boot from the OSX installation disk (It's the snow leopard disk which came in the Mac Box Set, which I believe came with the mac - because I don't have any other disks). I successfully get to the boot options menu by holding down Alt while booting - The OSX installation disk shows up correctly next to the internal hard drive. However, when I click on it, I go to a grey screen with the apple logo, then nothing else happens. I've left it like this for several hours.

2. I know the drive and the disk itself are not faulty because it loads fine when i'm already logged in. If I click 'Install OSX' from the disk this way, the installation proceeds as normal until the mac restarts, then I get stuck on the same screen as option 1.

3. I have also used disk utilities to create a bootable USB from the CD. Again, this USB shows up in the boot menu when I hold down alt, but when I select it I get stuck on the same screen again.

4. I understand OSX 10.7 onwards has a recovery partition that would allow me to bypass this issue and restore without the installation disks- however, i'm stuck on 10.6.8. I have a app store link to a free upgrade to 10.11 El Capitan which i've previously used on a similar age MBP. However, when I go to this link in the store, the 'Get' button under the item does nothing. Nothing downloads or installs.

Am I missing something, or is there any other way I can restore this iMac (aside from either buying new installation disks from Apple, or paying for the 10.7 Lion upgrade - neither of which are guaranteed to work either).

Appreciate any help you can offer. Thanks!