Selling a MacBook Pro with preinstalled OSX and transferring the license
With the system I've updated, will the buyer be asked for his apple ID and will that replace mine?


When they first use the system and create a User ID etc, they will be asked for an Apple ID (which is not entirely mandatory) and it will have no knowledge of you as the previous owner, nor will it care what ID is used.


If that's not the case, will he be able to do updates without having the password to my account?


Updates will be possible without having any knowledge of the ID that originally purchased it


Can I change the Apple ID that owns mavericks to him, even if I have to gift him Mavericks?


No, purchases via the Mac App Store are tied to your ID, and cannot be transferred.  Also, you cannot gift a free app.


Is there a better way to do all this?


Not really, you should just advise the purchaser to "buy" (It's free, so no cost) a copy of Mavericks against his own ID, such that it can be used for restores etc in future and to ensure that he owns a copy legitimately.  He should be able to purchase it on a machine that is already running it, simply by logging into the App store with an ID that has not already purchased it.  It may kick off a pointless download which can be cancelled, but it will get him square from a licensing terms perspective, and from a safety/recoverability perspective.