Migrating from one Macbook to another I am planning on selling my Macbook Pro and buying a new Macbook Retina, however I'd like to sell my current one first before I buy the new laptop, so I need a way of migrating all my stuff from my current laptop to the new one without having the new one available at the time of sale of the current one.

I know it's easy to migrate from one laptop to another provided you have both in front of you, however since there will be a few days between selling the current and buying the new one, I'd like to migrate using an external drive as an intermediary, is it possible? What software could I use?
You can use builtin Time Machine application.
First - make a backup on Your external drive using TimeMachine.
Then on the new Mac use Migration Assistant.

Here is the link with detailed info: http://www.macworld.com/article/2066996/how-to-transfer-backups-from-time-machine-to-a-new-mac.html 


  On the new iMac make sure that you have access to the drive where the Time Machine backup is stored and then launch Migration Assistant. It will notify you that it needs to quit all open applications. Allow it to do so.
  The first Migration Assistant screen will offer three options regarding data transfer. Select the first one, which reads From a Mac, Time Machine backup, or startup disk and click Continue.
  In the next screen choose the drive that holds your backup and then click Continue.