SecondMarket could make even more money if they had a private source that would sell them bitcoins below market price. That difference in price would be their extra profit.
They do.
I don't know how KNC sells their mined coins; but they seem to be a carefully planned enterprise, so I guess that they secured from the start a contract with some investor who is bound to buy their bitcoins at a fixed price. If that price is still below market, that could be a source of cheap coins.
SecondMarket of course will not
knowingly buy stolen bitcoins. But why would not they buy at 80%
discount of market price from some respectable investor who bought them at 60% from someone who bought them at 40% from someone who bought them at 25% from a Russian mobster who bought them at 15% from the MtGOX thief? It is not their job to trace all stolen coins in the blockchain, is it?
EDIT: "80% of market price", of course, not "80% discount".