Q: Can I exchange my Mac book then add some money to get a new Mac book?

You can sell your MacBook to a used Mac dealer for cash and possible trade in credit. There are some ideas and links on this previous thread.

https://discussions.apple.com/message/21976603#21976603

Just a notation that I saw a free standing machine in a Mall recently that offered cash for iPhones, iPads, iPods and other brands of similar hardware. There were people using it so I haven't been able to check it out closely. I suppose laptops could be included sometime.