Sell MacBook Pro 

After you have created a backup of the files on your Macbook Pro, you will want to make sure that you sign out of iTunes. To do so, open iTunes. On the menu bar at the top of the screen, choose “store”. Next, choose Deauthorise this Computer. Once you have entered your Apple ID and password, you should click Deauthorise to complete the process of signing out of iTunes.
When you deauthorise your computer, you are removing access to the content you purchased from iTunes while you owned your Macbook Pro. This means that the next person who receives your Macbook Pro will not be able to access any of your movies, music, books, or other content so it is a pretty important step to complete.

Before starting, you will need to archive or make copies of your iCloud data if you use any iCloud features on your Macbook Pro like Find my Mac.
The next step is to go to the Apple menu and select System Preferences. Then click iCloud and uncheck the Find My Mac checkbox. Lastly, sign out of iCloud. Go to System Preferences where you should click iCloud and sign out. You will be asked whether you want to remove iCloud data from your Macbook.
Remember, when you sign out of iCloud, you are not removing data from your iCloud account. You are only removing data from your Macbook Pro so the next owner will not have access to it. Your iCloud data will remain safe in the cloud and you will be able to access it again once you log into another device using the same Apple ID.

This is a short, easy step to be taken if your Macbook Pro is using the OS X Mountain Lion or later. If not, you will not have to do this step.
If you are using the OS X Mountain Lion or later, go to the Messages app and click on Preferences. Next, select accounts and select your iMessage account. Finally, click Sign Out.

This is the final step you should take if you are selling your Macbook Pro. Before you start, make sure your computer is connected to the Internet.
Restart your Macbook Pro. When you hear the startup sound, hold Command and R keys to go to OS X Recovery. Then, select Disk Utility and Continue. Select the name of your startup disk and click the Erase tab. To erase the drive, click Security Options and choose an erase method before clicking OK.
When you see the format pop-up menu, choose Mac OS Extended. Type a disk name and then click Erase. After the drive has been erased, you can close the Disk Utility window.
Next, choose a network from the Wi-Fi menu if you are not connected to the Internet any longer. Choose the option Reinstall OS X. You can next click Continue, and follow the directions on the screen to finish reinstallation.