Trading in iPhone SE for MacBook accessories credit

I have a used iPhone SE. I would like to buy a Magic Trackpad for my Macbook air. I already have a gift card for a MacBook that I traded in last year. Can I trade in my iPhone SE and obtain credit that I can add up to the gift card I already have? Then can I use both my previous gift card and the new credit obtained through this iPhone SE to buy a Magic TrackPad?

[Re-Titled by Moderator]