No you can’t unfortunately. The 30 day free subscription is for all new accounts, and if you bought your copy second hand you won’t be able to get that, due to the fact a new account was most probably made by the previous owner.
To utilise the free subscription you need to create a new account which means when you enter the registration key for FFXI (and it’s subsequent expansion’s, as they too have registration keys.), that copy is then locked to that account.
If you still want to play this game, you’ll have to try and get a new copy either from a game store or go and buy it online from Square’s website (http://www.playonline.com).
I’m guessing by the wear on the underside of the discs they’re well used though. Oh well… one for the shelf. At least I dont gotta waste another 30 mins ebay listing it =)