apple trade in - How does it work?

When buying, click the Get Started as seen in the screenshot example below. Follow the prompts through the rest of the trade and buying process. I don't remember the exact wording but that "Get Started" prompt leads to getting the estimate, a question of whether you want to trade and will go on through the rest of the buying process. The check out screen and your order confirmation will show that you are doing a trade in and will show the estimated value. There is also information in the Frequently Asked Questions section of Apple Trade In - Apple

Your order confirmation should have a small section that shows your trade-in estimate. About 4 or 5 days later you should get another email saying your trade in is "in the works." They don't ship the trade box until shortly after your new Mac ships.