The apps for most streamers also have a volume control. My bedroom setup has a remote. I use it for off/on only. A WiiM mini feeds it tunes. Just leave your receiver on and set the volume. As they used to say in radio, just set your station here and rip the knob off.

Found out the new echo’s don’t have line out.

Sorry about your $ issues. A local vintage shop sold some restored AU models for $700-800 each. eBay will take their cut of your profit plus shipping and state tax. So your $700 with free shipping ($50 shipping) will maybe net you $575 or less. CL will get you a bunch of scammers and low ball offers. Or worse somebody who shows up to buy then haggles. Hopefully you will be in better shape shortly. Later you will regret selling this.