Small Business Employee. Is this legal? ?

I work at the small business run by one owner. He hired me pretty much on the spot because knowing what i know now no one else would bite at his craigslist ad. Okay a few things when he hired me instead of signing a w-4 i signed a w-9. I've never had to sign anything like that before. 2nd I'm from Tx and get 65/day @ 6.50/hr. 10 hrs/day 6 days/wk. 7.25 is min wage. Is there a loophole my boss found that allows him to underpay and over work me? i am also the only one here all day and hardly make any sales.. so i pretty much run the store. He also treats me terrible. not to complain TOO much. What can u guys tell me to clarify my siutation? Is this legal? Can I take action or is this how it is?