When buying a "Used" or even some "New" games from EA Sports, be EXTRA careful to make sure the game's never been registered.  If the ad doesn't say specifically it's never been registered, WRITE the seller and ask.  The reason for this is that if the game has been registered, you CAN'T play it online unless the seller gives you the user name and password used to register the game.

I bought a copy of Tiger Woods PGA Tour 07, and found out that while it had not been played online, it HAD been registered (that's one of the first things they ask you to do when installing it), and so couldn't be used online with my usual EA account.  The seller was great and refunded my money, but it was a lesson for both of us.

I wrote EA and asked about reregistering a game....here's their response:

"If you have purchased a previously registered copy of  "Tiger Woods PGA Tour07" you will not be able to create a brand new account. You will need to get the user name and the password from the person that sold the game to you. If you do not have the user name and password, or the registration code for your copy of the game, then we will not be able to assist you. We do not recommend that you purchase used software or accounts off the Internet, unless you are sure that you are buying a brand new game which has never been registered."

So, be careful and enjoy the games!