There's a good book by Kevin Poulsen called "The Kingpin: How one Hacker Took Over the Billion-Dollar Cybercrime Underground" that is a bit out of date at this point (2011), but it goes into great length on all of the dynamics of the early forums where all of carding/spam/botnet operators did business.

In a forum/marketplace like this, your reputation is worth a lot of money. And if you scam someone and get banned, sure, you can just join again under a new identity, but building your reputation up again means you will lose out on a lot of potential sales.