The problem is Apple lowered the older devices prices new significantly. That of course drives resale of those used devices lower if you can get new for say $50-100 more. An 8+ brand new is $699 for 64gb right now.
So yeh you paid $799 for it, but is someone going to buy a used one for $650 or new with warranty for $699? The market is just not good for used older models unless they're cheap. And you can't add Applecare to a used phone either so if it breaks you're SOL full price repairs.
Personally, if I had to choose a used $600 8+ (treated who knows how not seeing it person or its charge cycles on the battery) or $699 new Id go new still. The extra $99 is piece of mind its brand new with a warranty and able to pick up Applecare+ still.
It's also being squeezed down by a much more advance XR phone with a bigger display and better internals at $749.