If you had already finished the buying and signed the sale agreement, then there is nothing that you can do. You just need to take the car to a mechanic and pay for its repair. You cannot blame the person that sold it to you because you believed what he had described his car. However, you can also seek legal advice from an attorney so as to be offered professional advice.
caveat emptor. let the buyer beware. you could take him to court cost$ and may lose. Did he imply a warranty? if not you're screwed. just find the leak and fix it. chances are it's the windshield. remove molding. cut under old gasket gently push out windshield scape away old gasket replace gasket reset glass replace moldings. Or call safe lite.