Before buying digital games, you have to take into account if you are or not going to sell your console later on. In my case I would never sell my console so the digital games are fine for me. And besides, you may not be able to sell these games, but you never really lose them, so it's a pro right there IF you want to re-obtain another PS3.

Do you know how I can deactivate my account (on my current PS3 only)?

Now I've deactivated the game content. But it still automatically signs in to my account if I turn it on. Do you know how to prevent this, or even better; how to remove the user itself?

If you remove the user, you lose access to games.
I don't know exactly where, but there should be an option to disable automatic login.

Although, if you had done what I have written up there, you wouldn't be able to log in anyway because of the changed password.

I honestly recommend you do what I suggested, it is the best solution.

You want to deactivate movie and music content also.  If you have multiple users on your PS3 you want to deactivate all of them, not just the one that you use as your main. 

Go all the way left to your users.  It'll have an arrow below the name of whatever user is signed in.  Press triangle to log out and go to the PSN to sign back in and uncheck auto login and delete your password.  Or you can just delete the user name whenever you hit triangle. 

Thanks for helping, I did what you suggested. It just seemed more complicated than it actually was.

No problem, glad I could be of help. If everything was done right, all games on the drive should work for the new user.

You can try it out by using a different account to start the games, or just let the buyer test it out so he can be sure that the games are his to enjoy.

Also, since you've done it over PC, you can use your account again on another console, and redownload those games again, and use them for yourself (so even though buyer can play them, you can still play them also).