You could try kbb cash offer and see what their computer spits out, let's say it says, $69, you can then contact the dealers that they'll show you are willing to pay the $69 and tell them you want $75. It's what I did with my RAM and worked out perfect.
Other places to try are carvana...
One on BAT sold for $275k LAST MONTH and another one is active right now with a day or more left on the auction and it's currently sitting at $212.
https://bringatrailer.com/listing/2022-gmc-hummer-ev-edition-1-2/