1) if no society is formed builder would demand transfer charges
2) did you not obtain NOC from builder before purchase of flat ?
3) the builder should have given you no due certificate and then only you should have purchased the flat
4)you can obtain in writing from builder what are the balance dues payable .
5) contact the seller and ask him to clear the dues as per indemnity clause in your sale deed
6) if seller refuses to pay then you pay the dues to builder take possession of the flat and sue the seller to recover money paid by you