How can I upgrade Mac OS X for free on a recently bought rMBP?
Apple typically calls their upgrade program up to date.

In this case, you would go to this page below and follow the instructions based on the purchase date of your Mac.

http://www.apple.com/osx/uptodate/

Your seller can assist as well - but most people can find things themselves once they know where to look. You can then use one Apple ID on the Mac App store to redeem the Mountain Lion upgrade just like you may have to redeem the iLife (iPhoto, iMovie, GarageBand).