How do I sell an unopened Macbook Pro? My old MBP broke, which Apple replaced for me. By the time they decided to replace it, I had bought another laptop.

I want to sell the unopened MBP that Apple gave me.

Is there anything I need to be aware of when selling the MBP to something?

For example, is it possible the MBP has been registered with my existing Apple ID, and that I have to unregister it? How do I check?

Also, when advertising the MBP, is it safe to tell potential buyers the serial number?
Personally, I would feel odd buying a laptop from a third-party that is still in the (purportedly) original shrink-wrap. I would take the MBP out of the box, power it up and see that it's fully functional, and then advertise it as "barely used" and "like new." And yes, you need to provide the serial number.