can some one explain to me how this trade in offer for wii U works?

[Metrium]

You will quickly notice that the games that have a 8$ value are fairly recent big blockbuster games like Bioshock Infinite or Far Cry3. So forget about trading in your really old crappy games. Most of them are games that you will have a hard time departing from and won't believe that their value is only 8$.

But if you manage to only 5 of those, you get 100$, so it's still great.