Just had the same thing happen to me. Traded in an iPhone 6S. Original battery, so of course would not hold a charge for long. Promotion was $400 trade-in value but got $20 for a phone with an "unresponsive screen." I mean, did they bother to charge it? Verizon rep was unable to adjust, or offer any other promotion. I am unable to return the phone at this point, it's been over 14 days. Did not need the phone immediately, could have waited for a different deal, but am stuck paying full price - who does that?? - for the phone.
The offer was so deceptive. The range of phones "eligible" was huge, with no breakdown on the maximum value per device type. For example, I would not have expected $400 for an iPhone 6S with an original battery - yet there was NOTHING to indicate that I would get less, unless the phone was physically damaged. It wasn't.
Verizon needs to make this right. Even if I had photos/video of the phone, I was given no way to challenge the valuation.