They sell cards at fixed rates similar to other proxy sites, but the cards are priced on a per-deck basis
Dimir didnt get many cube goodies in Gatecrash and Dragons Maze (Far is sweet but not spectacular, Duskmantle Seer is sweet but bad, and Nightveil Specter is a devotion enabler more than it is a blue/black card), so hopefully cubes everywhere get some sweet new goodies like Azorius and and Izzet got last time
You can also modify those beginner decks as you get more pieces to make your deck more competitive
While you dont need the in-depth knowledge of a judge, you should be both familiar with and up to date on the following aspects of Magic: The Gathering : Rules of how to play The sanctioned formats promoted by WotC (Commander, Standard, Pioneer, etc.) Top decks, strategies, and cards within sanctioned formats This will help you not only understand why certain cards are desirable compared to others, it will also hone your instinct for what might make a good card in the future