Selling back to apple

Apple accepts trade-in of qualifying devices - either in-store at a physical Apple Store, or using the online trade-in process:

Apple Trade In - Apple

Each device is assessed its own right. If using the online trade-in, you will be sent a shipping kit for each device. Devices are returned, assessed and a credit value assigned. The trade-in value is returned to you by way of an Apple gift card for each device - and these can be redeemed against a future purchase with Apple, or Apple subscription services (e.g., Apple Music, News, iCloud etc.) Trade-in values are not returned as cash.

If a trade-in is initiated during an online purchase, you will usually pay full price for the item - but will receive a refund against your credit card upon subsequent return (and acceptance) of the item that was returned for trade-in.

I believe they only offer you credit to purchase other stuff

but you can read about their trade in system here

Apple Trade In - Apple