And you seemed to miss the point that, for some people, the convenience is worth the lower payout for cash now. Many people prefer an immediate, but smaller payout over a larger payout that requires more time and effort (selling online, figuring a price, dealing with shipping, etc.). Yeah, technically you may have "lost" money (you're not getting the maximum value for the game), but money you can use immediately might be worth more to a person than a disc that just takes up space and collects dust. It's all about personal priorities.
If all vendors bartered for video game, or if game discs were a form of currency, you might have a point. Sadly, we live in the real world.