No I would not take the chance, you want the warranty, and would you still be eligible for the upgrade to HDMI 2.1 if you wanted?
Did the seller even register his unit, if not you may be able to if he is able to provide a receipt and proof of purchase?
You can also look to buy a used demo unit from a an authorized dealer and see if they can honor the warranty.
My dealer where I bought my AVM70 from has a used/trade in unit on his website
www.simply-stereo.com
I have bought a few items from their shop and they do a solid job with after sales support.