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.
How do we know this?
For one thing, you can call them up and buy/sell to them in 25BTC minimum lots. However, I think these channels are low volume right now simply because SecondMarket purchasing is loosely correlated with rises in the exchanges. And I'm guessing that bitpay isn't doing 1K btc per day right now... less spending more hoarding during the slump I would guess.