I differ with Mr. Rajeev S Vadrali's opinion partially:
Any legal action, original suit or arbitration, if there is such a clause in the agreement, has to be initiated within three years from the date of agreement. If a time for performance is stipulated say as 1/2/3... months/---days, then the limitation gets extended by such period.
(As for stamp paper, it is perpetually valid;in your case the issue is academic, as the same hasbeen utilised for drawing the agreement in question)
If there is no automatic cancellation of agreement clause after a specified period, in the agreement, then the agreement is still valid upto the date of aforesaid limitation.
If you have issued notice of revocation and cancellation of agreement, with intimation of forefeiture of advance received, if permissible under the agreement, or expressing readiness to refund the whole/part of the advance amount, if such a case is warranted as per the agreement, then the notice by the 'purchaser' is subsequent and loses its value. Lest the said notice is valid, in spiteof thefact that the prescribed period for conclusion is exceeded.
Since you are not interested to sell, as evident from your query, you need not take any legal action. Let there be a reply to the notice, denying the contentions of the 'purchaser' and also repudiating the agreement on grounds of effulux of time, as well as mentioning forfeiture/refund as stated above.
The person who files the suit has to pay the court fee and also prove his case.If there is any deficit in the stamp duty, (since most states have made the stamp duty on agreement to sell ad-valorem = according to value), then you can raise a preliminarey issue of admissibility of the document in evidence, impounding of the document etc, during trial.
With so many gaps in information, more specific guidance is not possible.