Trade in my old phone to my husband for a new phone
Can my husband use my old iPhone 12 Pro Max as a trade in for him on the new iPhone 15 pro max
That shouldn't be an issue as long as the phone is in good condition and you prepare it properly for a trade in. This link explains how to prepare an iPhone for trade or to be sold or given to someone --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support