How to dispute a revised trade-in value for an undamaged iPad?

I traded-in my old absolutely functional iPad for a new one. I received an email informing "Our trade-in partner has inspected your device and has confirmed your trade-in value. Your trade-in will be completed soon and there is nothing more that you need to do.

Estimated trade-in value. $50.00 ( I have a screen shot of this status)

But next I get another mail informing me that the trade-in value has been revised to $0.00 , reason "LCD display burn in damage/streaks/discoloration across the screen"

My ipad was absolutely fine with not a scratch. I was upgrading because it was iPad 6th gen with 32 GB memory. I think they damaged it during inspection. I wish I had never sent it. So Frustrating.

What can I do now ?

[Re-Titled by Moderator]