how will i know about my mac trade-in value

To get more information about the "Apple Trade In" service click on the link --> https://www.apple.com/shop/trade-in

- Don't miss the FAQ at the very bottom.

- Start a trade-in by clicking on a device in the section "Select your device to get a trade-in estimate" - Smartphone, Tablet, Computer, Watch, Other devices (recycle only)

This is how much Apple will possibly offer you for your computer, not how much somebody would be willing to pay you. For that you should compare your computer to similar models for sale in your area using various web sites or a newspaper or a bulletin board in a coffeeshop, etc.