My most educated guess is that they are selling used games at those prices. I was looking into video game distributors and there are links on xbox's official microsoft page. They were very upfront about wholesale prices for new games. For games that sell 50 dollars retail the wholesale price is somewhere between 41-43 dollars. Video games are about selling in volume. That is why you never ever see a store that sells new games and consoles only. Every store is either a large retailer(walmart, target, bestbuy) or a small store that sells used games as well. Those stores make most of their money from the used games.
So if a site offers really low prices on newer games, they are either stolen, bootleg, or in a really used condition.