Can I trade in my macbook and get the new one for a cheaper price?

Not through Apple directly, Apple does not do trade-ins. There are other authorized sales agents that do such, but you'll get more money selling your computer yourself.

I was looking in to this at one time and found out you can't and that it's better to sell it on your own, but I do have a question for wjosten. If you have experience in selling a MBP on your own, where do you recommend posting it for sale and what has worked for you? Thanks.

I've always sold on eBay, or given my stuff to family members.