It is not sufficient if you send a legal notice telling your intention to cancel the sale as advised by Smt. Archana. You will have to actually cancel it saying 'agreement for sale is hereby cancelled'. You need not issue legal notice three four times. It is sufficient if you issue legal notice only once citing the clause which has enabled you to cancel the agreement and end it by saying 'therefore, you have cancelled the agreement of sale herewith'. You may forfeit the penal amount, if any mentioned in the agreement, for violation of the clauses of agreement.
Filing a case does not arise. After issuing legal notice by registered post acknowledgement due to the last known address of the buyer would be sufficient for you to go ahead and sell your property to anyone else.