Usually you will get a picture sent back to you that details the reason for the downgrade in value and you also have the option to have the device sent back to you. Cosmetic damages are subjective and will decrease the value even if the device works fine.
To avoid this process, take your device directly to the Apple Store for trade-in where you will get an on the spot value for your device and avoid the whole hassle of sending it in and then waiting to see what the actual value is. Of course, this depends on whether you have an Apple Store nearby, but depending on the location, a little more drive time may be worth it for a person if that is an option.