How to re-register a second hand macbook air??

First, the Mac needs to be unregistered from the old owner's account in order to register it. To check this, open http://supportprofile.apple.com, log in with your Apple ID, press "Add Product" and type the MacBook serial number. The site will give you the steps to find it.

If the website does not allow you to register your MacBook because it is still registered, you need to contact with the old owner in order to deauthorize it. He/she can do it through that website or by contacting with Apple > http://support.apple.com/kb/HE57

After that, register the Mac.

Register Apple Products > My Support Profile

pw168 wrote:

I bought a Macbook Air (June 2013Model) second hand..

Hopefully All these Steps were done by the Previous Owner Before you took possession.

Apple What to do before selling or giving away your Mac

http://support.apple.com/kb/HT5189?viewlocale=en_US&locale=en_US