all the landowners must have signed and registered development agreement in favour of builder
this agreement binds not just the owners during their lifetime but also their legal heirs (as can be seen from the title clause itself which would say 'this expression unless contrary or repugnant to the context or meaning thereof shall be deemed to mean and include the legal heirs, executors and administrators')
so even if any one of the land owner passes away, his legal heirs will be bound by the development agreement by and under which the landowner had consented to grant development rights to the builder concerning the owner's undivided share in the land
so it is incorrect for the registrar to insist for the life certificate of the landowners
also the sale agreement is between you and the builder and NOT between you and landowners
the duty of the registrar is to see that proper stamp duty is paid on the document and the parties who have signed the document are present before him to admit their signatures at the time of registration
by insisting for life certificate the registrar is clearly exceeding his jurisdiction
request the registrar to give reasons in writing for his refusal to register the sale agreement with clear justification for such refusal
on basis of that order you will have to file a writ petition in High Court to direct the registrar to register the sale agreement and also claim cost of the legal proceedings