Are you looking to buy this used? If so, do you have your own set of registration codes?
If you don't, make absolutely sure you get the seller's PlayOnline account with the copy of the game, and that it's an active account. If you get the game without a set of codes (4 total, see below), it'll be nothing but a paperweight, and likewise if you get an outdated account, as the registration codes will be invalid.
For reference, you'll need a code for each of these:
Final Fantasy XI: Online
Final Fantasy XI: Online: Rise of the Zilart
PlayOnline Viewer
Tetra Master Online