Well that's pretty simple to get around for Nintendo and make it harder than your average bear to crack.
Firstly, they can set an expiration date on games. So let's say you have a Wii and sold it with a bunch of games on it. Those games will disappear in 30 days.
Secondly, you verify the games online through your account anytime you want to push the expiration date back. This will most likely happen automatically anytime you are connected to a network and have once before connected to your account.
And thirdly, if you are actively connected to an online account and the automatic verification finds that you own a game that is not on your account, it could then delete it.
Easy
kinda.