Now that you know and understand how the Denon works, why wouldn't you just keep the Denon AVR-X2800H receiver. It's one of the best 7.1 receivers offered today.
Besides, good luck in finding an interested party to buy a now used receiver. Most people don't share our interest of this hobby. Those that could be interested, want to pay less than what a new soundbar would cost for everything, wire, speakers, and receiver.
I gave up trying to sell receivers. Back in the day in the late 90s and early 2000s, I knew many who would want my used, new looking gear. Now a days, no one is interested as this hobby has passed for cheaper, more personal options.
If you can sell it for what it's actually worth, Great. BTW, I grabbed my 'new' X2800H for $679 from Amazon on their Amazon day. Today they want $879 as of now.