I'm one of those that bought an original, 6" Aura off of eBay. Five years later it's still my daily device.
One warning, you've already been spoiled by the Clara HD's superior screen. Aside from the Glo HD or an Aura 2nd Edition, any Kobo 6" device is going to use a Pearl (or older) eink screen which is noticeably more gray. They will also be lower resolution and won't have the color changing light.
Like I said, I still love using my original Aura. It works fine for me. But cheap old Kobos won't look as pretty as your Clara HD. Buy an old Kobo. They get updates, just like the new devices. Software wise, the only difference is that Overdrive wasn't integrated (which seems weird to me). I just wanted to prepare you for the difference in screen quality.
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