bought a car here

I bought my Grand Prix here a few years ago, they fix the odometer, so the miles match Carmax records. I didn't notice until a few months into it. While on a road trip to Detroit, I set the trip meter to check my gas miles and after I reached Detroit the trip meter showed 160 miles instead of 300 miles. On my return trip the total miles from Detroit to Chicago in odometer was 140miles. After checking with my mechanic he told me the front dash has been opened and they have done some thing to ododmeter.When i called the owner with this issue, he was sounded off like i never bought the car here and when I went in person he asked me whats the guaranty that I didn't do this to the car, I would not suggest buying a car here. Go to a real dealer.