I imagine that he got scared that he would be caught by the feds, so instead of taking the profit over time and risking his ass, he decided that he was much safer just stealing the funds all at once.
The admin is continuing to operate the site and diverting the site's commission to pay everyone back.
The site rakes in millions of $ worth of BTC per month in commission. Nobody in their right mind would jeopardize that income for a measly 4000BTC.
Nobody in their right mind would risk a mandatory minimum life sentence to deal drugs for any amount of money.
They get paid out of the fee for listings being bought, regardless of what those listings are. If someone sells a pair of shoes on SR, the site gets paid a fee. The problem is that the admins see that silly money being tossed around and choose to get involved in the illicit sale of illegal material.
But to blanket SR admins with "They are drug dealers" isn't accurate at all. If I ran a shadow market, I wouldn't give two damns what people were selling. And I sure wouldn't be a drug dealer. I would just take my fee for each listing, not know or care what it was for, and be happy to provide a free market for ANY type of goods.
