http://en.wikipedia.org/wiki/Market_maker
http://en.wikipedia.org/wiki/Market_power
People move the price because they use the market. You don't need to use both markets, only one. You don't need to move the market, you wait. You can also do it for several hours. Also, you can reuse the money, it isn't spent.
Bitstamp had 17000 BTC volume today. Supposedly you are only on one side today (another exchange has a constantly deviated price in the same direction) and can control 5% of the market without impeding your operation. That means you can arbitrate 17000 / 2 / 20 = 42.5 BTC. Now you only work 8 hours, so divide by 3: 14.1 BTC.
For each of the about 14 BTC, you get an average 10 USD profit, so you get 140 USD profit today. Multiply this by the inverse of the declared profit of 0.8% and you get: 140 USD / 0.8% = 17500 USD total.
Is it far fetched that they had 17500 USD or 44 BTC from client funds?