If you can afford it, buy the phone straight out from apple, or whatever carrier you’re under. Then sell privately. Whatever apple, or whoever would offer you in the trade in is less than what you can get listing it online.

Since you have a 2 year old phone, it may be harder…as in you’ll have to play the waiting game. But as long as you’re not pricing the phone too high, you can make a sale.

OR

Disregard everything I just said and keep it simple. Trade it in to apple. They’ll give you the most for it. But you will have to pay for the phone in full, & then get refunded the trade in value once they have received and inspected your 11.