I don't have to pay electricity bills and currently have 6 machines running (2x 5870 each). Therefore as long as someone is willing to buy BTC, I make money

Not much per day, but it adds up

But SOMEONE is paying around $200 a month for your mining operation. Not all of us can (or will) steal that much electricity, especially without raising eyebrows/getting caught.
You must be in a college dorm or something. $200 a month is a lot, even for a business.