1) sale deed needs to be registered within period of 4 months
2) further period of 4 months is granted on payment of penalty .
3) in your case period of 6 years have passed. no registration can be done after period of 6 years .
4) unless sale deed is registered you dont have clear and marketable title to property .
5) If a deed of transfer, which is compulsorily registerable, is not registered it will not be admissible in evidence under section 49 of registration act
6) dont purchase the property