You bought a dog off Craig's list. Anyone selling dogs on Craig's list is not reputable at all and usually lie to people just to make money. Also, most people aren't even aware of what a "Pit Bull" is. There *is* such a thing as an American Pit Bull Terrier but the majority of dogs called Pit Bulls are just street bred mixes of them, American Staffordshire Terriers, American Bulldogs, Bull Terriers, Staffordshire Terriers, and sometimes Cane Corsos, Presa Canarios, and occasionally Labs, Rotties and hounds. Your basic "Pit Bull" bred by someone on the street or Craig's list--or the kind you find in a shelter (seriously if you ever want another Pittie go to a shelter, you can easily find every shelter full of them, including puppies) are really bully breed mixes. They typically have very different styles of body types, heights, weights, heads, muzzles, ears and well...everything except the basic blocky head, muscular body and long tail.
Your dog is very likely that street bred bully breed mix, on the taller/larger side. I don't think he's got any Mastiff but maybe see how big he is at full maturity.