When you first sign on to Playonline and FFXI you have to register your keys, so I'm assuming that if you buy a used copy of FFXI that the other person would have to unregister/delete their account for you to be able to play.

I know when I deleted my first character and then created a new one several months later I had to re-enter all of my FFXI keys, so I'm assuming that deleting any content ID's and Playonline ID's associated with those keys will allow another person to use them on another computer and play.