Re: where to sell or trade on BGG?
To sell it, you'll want to go to the GeekMarket and add a copy of the game for sale. You could sell the base game together with the two extra characters, or sell them separately. I'll leave it to you to research what you would prefer.
To trade it, you'll need to add the game to your collection, and mark it as "For Trade". If you also mark some games that you "Want In Trade" then you can go here to find matches for users who have what you want, and who want what you have.