Aloyds Negative

I bought an xbox console and a copy of Halo 3 from this guy that lives in my community because he was leaving for college and selling it really cheap ($150), so I get home and hook it all up, and it says that my console is permenantley banned from Halo or something, then something else pops up and says my account is banned too (I was using the account already on the xbox, I even put money on it to change the gamertag). How can I go about fixing this because I don't have money to go and buy more games and I have no way to contact this guy because he already left. Am I ******?