Apple Trade-in issue - how to dispute?

Hello, I traded in my 2020 M1 13" Macbook towards the new 16". My trade-in was in pristine condition as it was always in a case. I have been going round and round with Apple to dispute this devaluation as this cannot be accurate. I am flabbergasted and requested pictures, which they have been unable to provide.

I travel 3 weeks a month and have been trying to dispute this when home. Does anyone have a similar experience? Any pointers on disputing this further? I am kicking myself for not taking pics before sending!

Any feedback is appreciated!