Trying to hide a big scratch on iPhone 6S screen
Based on your included picture, I feel that it would be best to research whether T-Mobile accepts this type of return. On the other hand, I think that it would be better off if you just show it as it is, because if they find out that you have hidden a scratch, they might deduct cash. I think you should just trade it in as it is. Even so, http://www.t-mobile.com/cell-phone-trade-in.html states that your device's trade amount will be determined inside the store. So, just leave it as it is.