How does the trade in online work?

A few words of caution regarding the trade in process.

You will always receive more money by selling the product yourself.

If you do decide to do an online trade, be certain to take dozens of photos of the device, and record all information from the device prior to shipping it to the trade-in partner.

There is information in Apple Trade In - Apple

including but not limited to the following from the "How does it work?" section of that link:

Whether you’re making a purchase or not, we’ve made it easy to trade in an eligible device either in a store or online. Just answer a few questions regarding the brand, model, and condition of your device. We’ll provide an estimated trade-in value or a simple way to recycle it.

If you accept the trade-in estimate in the store, we’ll give you instant credit toward a purchase or a gift card you can use anytime.

If you accept the trade-in estimate online when you purchase a new Mac, iPhone, iPad, or Apple Watch, we’ll arrange for you to send us your old device. Once we receive it, we’ll inspect it and verify its condition. If everything checks out, we’ll credit your original purchase method and send you any remaining balance on an Apple Store Gift Card via email.

If you choose to trade in your old device online for a gift card, we’ll also arrange for you to send it to us. When everything checks out, we’ll send you an Apple Store Gift Card via email.

Regardless of how you choose to trade in, if the condition of your device doesn’t match what you described, a new estimated trade-in value will be provided. You’ll have the option to either accept this revised quote or reject it. If you reject it, your device will be returned to you at no charge.