How can I prevent my iPad from being undervalued during a trade-in?

You have 2 options for trade in. Either by mail or take it to the Apple Store. I recommend taking it to the Apple Store to get an immediate value and you won't have to go through the hassle of having to do it by mail. Using the mail in option you can only get an estimate until they actually see the device and many times the device will be devalued. You can take the device to the store for trade-in even if you purchased another device online.

Apple Trade In - Apple

I had the same issue for my iPad Pro 2nd Generation machine. I was quoted a value of $330 as it was in flawless condition. I received a Trade In update about a week later stating:

"Our trade-in partner has inspected your device and has confirmed your trade-in value. Your trade-in will be completed soon, and there’s nothing more that you need to do. Estimated trade-in value: $330.00".

A week after that missive, I received another email regarding my Trade In that stated:

"Apple Trade In

Please review your new trade-in value.

The condition of your device has been verified, and your trade-in value has been revised.

You have 14 days to accept the new trade-in value. If we don’t hear from you within 14 days, the trade-in will be canceled, and we will return your device.

Your device:

iPad Pro 11-inch (2nd Generation)

Serial/IMEI: DMPCG0UXXXXX

Estimated Trade-in Refund:

$330.00

Revised Trade-in Refund:

$0.00

Reason:

More than 13 imperfections on the enclosure/back glass OR dents larger than 5mm in length"

This is absolute BS, as the machine left my house in perfect condition. 2 hours on the phone with Apple Support yielded very little progress. I cannot locate the phone number/email address/physical location for the Apple Trade In Legal Department. Does anyone have this information?