Switchouse is a user-to-user broker like Half.com. They don't actually sell the videogames, only allow people looking to sell them a place to do so, which means that in the end, you'll have to deal with a possibly irresponsible human being.
Switchouse has had severe problems with user fraud in the past (I got shorted twice), and did nothing to help the defrauded users. If you do decide to do a transaction there, make sure you pick a seller with a good rating.
Personally, I'd just go to the B&M and pay the extra cash. The extra hassle isn't worth it unless your dealing with hard to find games.