The property is under cloud especially document wise. There is no sale deed registered on the seller's name. He is possessing only registered sale agreement document for which he has paid the stamp duty and not for the registered sale deed. Besides having all other ancillary supporting documents, the main document i.e., the title deed in his name is missing so he becomes ineligible to sell the property. Therefore it can be said that the seller is not having marketable title to sell the property and if yo buy this one without obtaining legal opinion you may land in a great trouble at a later stage.
Consult a local lawyer before making any decision or entering into a sale agreement.