removing previous owners info

I know this isn't what you really want:

Apple: What to do before selling or giving away your Mac - http://support.apple.com/kb/HT5189 or http://support.apple.com/kb/HT201065

How to Prepare your Mac for sale - http://www.thesafemac.com/how-to-prepare-your-mac-for-sale/

https://discussions.apple.com/thread/5655535

Linc Davis Feb. 2016 post on restoring computer to new condition - https://discussions.apple.com/thread/7447000?answerId=29726244022#29726244022

Realize even if you remove the AppleID there are still traces of the former owner all over the computer, particularly if somebody is knowledgeable to go poking around. Does your friend trust your skills enough to root out all traces of his ownership?

Even if it was a bargain with "additional software", is that software licensed? Unfortunately with the new way of distributing software through downloads, transfer of licenses is not the way it used to be where you sold somebody a set of CDs and the software was theirs. The software tends to be linked to an individual or at least their ID.

You should create a new OS X user account on the Mac, instead of continuing to use his existing user account. Go to System Preferences Users & Groups pane and add a new user account. Set up up as Administrator, not Standard. You can also click Login Options and set your new user account for automatic login. Log out and log in to your new user account. Set it up to link to your Apple ID. The new user account essentially makes the Mac a new Mac for you, without erasing it and reinstalling all the software. When you run iTunes, you are using a brand new empty iTunes library, that you can associate with your Apple ID.

Eventually, delete the old user account.