Nope, I would not buy a used receiver or a demo. The problem is Denon says there is a warranty but in reality you may never get it fixed. I sent my Denon 4500 in for repair 6 weeks ago and still have not heard anything from PANURGYOEM except they do not have parts.
Thats not much of a warranty.
If you want a deal get a new Denon 3600 for $799. That's less than the refurb price. This won't last.
9.2-channel home theater receiver with Wi-Fi®, Bluetooth®, Apple® AirPlay® 2, and Amazon Alexa compatibility
www.crutchfield.com