They are responsible for safety and preservation of all registered documents. If your question is on verification / Authenticity of earlier link deeds connected with such sale deeds referred in Present /disputed sale deeds, Certainly it is never responsibility of Sub-Registrar.
Buyer Beware is the law applicable in every case. Sub-Registrar's responsibility is confined to only collection of correct Registration fees and that the deed contains main stipulations without ambiguity, which is different from verification/authenticity.. Sub registrar is concerned only with format of such deed alone.
(Realizing this flaw, some state governments like AP, has created a website and any citizen can have access to EC and sale deeds on line, and the system shall not permit those Government and other properties where registration is not permitted)