The gist: the site lets you buy BTC with credit cards, at a substantial markup to the market price for Bitcoin (10 to 20%). The actual mechanism is by buying wish listed items on Amazon picked by the BTC "sellers." This lets the sellers use BTC on Amazon and also get a discount. The site keeps a spread.
Why is this a carder paradise? Because the only reason to ship a random address a $300 camera for $240 of consideration is the $300 you paid is not your $300. When the chargeback/fraud investigation comes in, the carder doesn't have the stolen goods and has no IRL nexus to the crime. They just have their illicit profits laundered to BTC.