Problem preparing ’08 iMac for resale

My wife owns an early 2008 iMac. She is the third owner, having bought it from her brother, who bought it from a previous employer. It had Yosemite on it when she bought it, but presumably it originally shipped with Leopard, judging by the OS X timeline.

She now wants to sell it so I am trying to prepare it for that, but I'm having difficulties reinstalling a pristine copy of OS X that isn't associated with her or my Apple ID, probably due to my ill-advised meddling with it.

Here's what I did after she got it:

• Upgraded Yosemite to El Capitan
• Realised that the Recovery partition would install a much older version, so thought it would be a good idea to update that as well, so:
• Made an El Capitan USB installer using the official method, completely erased the hard disk (including the Recovery partition) and installed the system afresh from the USB.

Now, when I attempt to use Internet Recovery, it runs the El Capitan installation, but prompts for my Apple ID before it will proceed. This is not a satisfactory state to leave it in; we can't sell it to someone with the ability to use Internet Recovery dependant on knowing the previous owner's Apple ID.

I suspect I need to downgrade the system (including the Recovery partition) to an older version of OS X that isn't linked to my Apple ID, maybe all the way back to Leopard. Then, the new owner can upgrade it to El Capitan again using their own Apple ID. However, the Leopard DVD originally shipped with the machine is long since lost.

So I have two questions:

1. Is attempting to downgrade back to an older version the right approach here?
2. How might I obtain an older version of OS X, preferably without needing to purchase it?