Wondergoo? You in Tohoku? Reminds of when I used to live in Akita, I bought Romancing Saga there.
Ahem, you just take the games you want to sell to anyplace that buys and be like " I want to sell these" "これを売りたいんですけど "kore wo uritain desukedo" like that, they take 'em, scan the barcode, check the disc, give you a price. you have to feel out a form usually, name, address, and work, then they'll copy your gaijin card and thats it.
Just be careful, some games drop in price reaaaaaaaal fast. RPG's drop fast usually, I mean like to 1000 yen... kind of makes it not worth selling. I just sold Shadows of the collossus yesterday for 2500 yen after spending 3000 to buy it used though, that was cool