Refurbishing MacBooks
The Software License Agreement for Mac OS X states the following:


  3. Transfer
  
  A. If you obtained the Apple Software preinstalled on Apple-branded hardware, you may make a one-time permanent transfer of all of your license rights to the Apple Software (in its original form as provided by Apple) to another party, provided that: (i) the Apple Software is transferred together with your Apple-branded hardware; (ii) the transfer must include all of the Apple Software, including all its component parts, printed materials and this License; (iii) you do not retain any copies of the Apple Software, full or partial, including copies stored on a computer or other storage device; and (iv) the party receiving the Apple Software accepts the terms and conditions of this License. For purposes of this License, if Apple provides an update (e.g., version 10.8 to 10.8.1) to the Apple Software, the update is considered part of the Apple Software and may not be transferred separately from the pre-update version of the Apple Software.


This means that a transfer to you as a buyer or to the next seller that does not meet these conditions (eg. does not include the installation media), does not imply a valid license transfer for Mac OS X for you to reinstall the OS or sell the computer again.