I got a good deal with a few games that I didn't have a week ago so I wanted to comment a bit. I think this is a great new source of games especially if you don't mind extras of stuff you already have. I am happy to dump my extras for free to a local store called gamers clique so its cool.
There are a couple of points to watch out for though. The shipping is pretty high, as in they are high to be charging what they are. They add on some sort of shipping fee as well so that makes it a bit worse. Also you are dealing with people who have no idea how to set up an Atari. You will see commodore 64, Atari 7800 and Atari 5200 carts in a 2600 auction. If you don't mind that then its OK. The second issue with this is that they have no idea how to verify the products are working. You are buying with no real assurances of quality which isn't great.
All in All I think it's a great option for new collectors and people looking to build up extra stock. If I were a store looking to expand inventory i would totally go this route.